This study explores institutional divergence and policy evolution in the post-Brexit United Kingdom, with a focus on Northern Ireland’s distinctive governance. It analyzes how Brexit reshaped regulatory alignment, trade policy, and intergovernmental coordination, emphasizing the consequences of divergence between Northern Ireland and Great Britain for policy coherence. Grounded in New Institutionalism, the research highlights the influence of institutions, regulations, practices, and historical legacies on governance outcomes. Using a comparative qualitative design, documentary evidence was drawn from legal texts, parliamentary proceedings, policy reports, and scholarly literature. Findings show that Brexit placed Northern Ireland in a unique regulatory position through the Protocol on Ireland/Northern Ireland and the Windsor Framework, introducing mechanisms such as green- and red-lane trade arrangements, democratic consent procedures, regulatory harmonization, and revisions to EU-derived legislation. These changes have generated regulatory fragmentation, political contention, trade sensitivities, and greater complexity in multi-level governance. The study recommends closer UK–EU–Northern Ireland collaboration, transparent management of Northern Ireland’s special status, improved intergovernmental consultation, flexible regulatory coordination, and inclusive policy negotiation to reduce uncertainty and support coherent governance in the post-Brexit era.

The post-Brexit era has sparked significant debate over the nature of governance and institutional arrangements in the United Kingdom, particularly in Northern Ireland. At the heart of this discussion lies the concept of institutional divergence, which refers to the process by which political and regulatory institutions evolve in different directions, creating variations in rules, practices, and governance structures across regions (Hantrais, 2017). Closely linked to policy evolution is the gradual transformation of public policies in response to shifting political, economic, and social contexts (Murray & Robb, 2023). In the UK, Brexit has acted as a catalyst for both reshaping regulatory alignment, trade arrangements, and intergovernmental relations. Northern Ireland’s unique position under the Protocol on Ireland/Northern Ireland and the Windsor Framework exemplifies how institutional divergence manifests in practice, producing distinct governance outcomes compared to Great Britain. To analyze these dynamics, this study draws on contemporary political theories—particularly New Institutionalism, which emphasizes the enduring influence of institutions, historical legacies, and regulatory frameworks on political behavior and policy outcomes. By situating Brexit within this theoretical lens, the research highlights how divergence and evolution interact to shape the coherence, complexity, and contestation of governance in the post-Brexit United Kingdom.

Brexit represents one of the most consequential political transformations in 21st-century Europe, reshaping the relationship between the United Kingdom and the European Union and producing far-reaching institutional consequences (Igwe, 2022). Globally, the withdrawal of states from integrated regional frameworks often triggers debates over institutional divergence and policy adaptation, as governments recalibrate the legal, economic, and political systems embedded in supranational orders (Wolff & Piquet, 2022). Brexit thus offers a distinctive case for examining how disengagement from established institutions catalyzes policy change. Institutional divergence refers to the gradual differentiation of political systems as they develop distinct rules, norms, and decision-making processes (Wolff & Piquet, 2022). In the UK, this process is evident in the disentanglement of domestic law and regulation from the EU’s acquis communautaire, reflecting broader dynamics of de-Europeanization and renewed assertions of sovereignty. Institutional divergence unfolds on multiple levels: globally, as states exit supranational frameworks; continentally, as Europe adapts to such departures; and regionally, as substate governance units recalibrate policies in response to shifting mandates (Wolff & Piquet, 2022). Brexit exemplifies this layered divergence.

Northern Ireland occupies a unique position in the post-Brexit settlement because the Protocol on Ireland/Northern Ireland maintains partial alignment with the EU single market for goods (Whitten, 2024). This arrangement has produced internal divergence within the UK, as Northern Ireland’s institutional and market frameworks differ from those in Great Britain. The Protocol creates a hybrid legal and policy space designed to prevent a hard border on the island of Ireland while preserving UK territorial integrity, but it has also generated political tensions and divergent policy paths between devolved institutions and Westminster. The “Global Britain” agenda illustrates how the UK is redefining its external economic role outside EU trade agreements, emphasizing regulatory autonomy and global market engagement (Sowels, 2023). Yet divergence also introduces significant policy challenges. Without binding alignment to EU standards, questions of market access, regulatory equivalence, and legal coexistence in goods and services regulations become pressing (Howell, 2020). These challenges are especially acute in Northern Ireland, where unique governance arrangements shape local politics. Unionist parties argue that the Protocol entrenches asymmetrical divergence from the rest of the UK, complicates internal politics, and undermines policy coherence (Kelly & Tannam, 2023).

Policy evolution in devolved contexts can be traced through legislative amendments, alignment choices, and institutional negotiations over frameworks such as the Windsor Framework, which revised aspects of the Protocol (Whitten, 2024). At the global level, Brexit has also driven regulatory divergence, with UK environmental standards increasingly departing from EU norms—raising concerns about potential rollbacks in areas such as pollution control and chemical regulation (The Guardian, 2024). Within the UK, devolved administrations have pursued distinct strategies: Scotland and Wales often seek continued alignment with EU norms, while Northern Ireland remains largely governed by the Protocol (Birrell & Gray, 2017). Taken together, regulatory fragmentation, political contestation, trade disruptions, environmental risks, and multi-level governance challenges constrain policy development in post-Brexit Northern Ireland. Tracking divergence through indicators such as regulatory alignment, legislative amendments, consent decisions, and framework negotiations provides policymakers with practical tools to monitor gaps and coordinate responses.

This study is therefore motivated by the need to systematically analyze these multi-layered processes, treating institutional divergence as a structural force and policy evolution as its observable outcome. The case of Northern Ireland is especially significant, as divergence intersects with peacebuilding frameworks and cross-border cooperation. By grounding the analysis in empirical evidence—such as regulatory alignment, legislative consent decisions, and negotiated frameworks—this research clarifies the relationship between institutional divergence and policy evolution in a post-Brexit context. Ultimately, this study explores two central questions: How has institutional divergence between Northern Ireland and Great Britain influenced policy coherence and evolution in post-Brexit governance? And what role do measures of institutional divergence play in mitigating trade, environmental, and governance challenges in Northern Ireland? By addressing these questions, the research contributes to scholarship on devolution, regional governance, and supranational withdrawal, while offering insights for policymakers navigating the UK’s evolving constitutional framework.

Institutional Divergence in Post-Brexit Governance: The state of LITERATURE

Institutional divergence refers to the process by which political, legal, and administrative institutions gradually adopt distinct rules, norms, procedures, and policy directions after previously operating within a shared governance framework. In the post‑Brexit context, Wolff and Piquet (2022) conceptualize institutional divergence through the lens of Europeanisation. They argue that the United Kingdom’s withdrawal from the European Union should not be understood as a sudden rupture, but rather as a gradual and layered transformation involving phases of de‑Europeanisation, disengagement, re‑engagement, and continued engagement. Divergence, therefore, is not simply a break from the EU; it represents a stepwise reconfiguration of policy rules, institutional relationships, and governance practices that were long shaped by EU membership. Igwe (2022) reinforces this perspective by framing Brexit as a political project aimed at “taking back control.” From this vantage point, institutional divergence is closely tied to sovereignty, national autonomy, and the reclamation of decision‑making authority from supranational structures. Together, these accounts highlight that Brexit is best understood not as an endpoint, but as an evolving process of institutional differentiation—one that redefines the UK’s governance trajectory while reshaping its relationship with Europe. Institutional divergence also manifests in regulatory and constitutional differentiation. Howell (2020) observes that regulatory divergence raises critical questions about whether the UK should continue to “shadow” EU law or carve out space for domestic regulatory variation, particularly in sensitive sectors such as financial services. Birrell and Gray (2017) extend this perspective by emphasizing how Brexit interacts with devolution and multi‑level governance. They highlight that Scotland, Wales, and Northern Ireland experienced Brexit through distinct institutional lenses, shaped by their varying relationships with EU programs, regulations, and decision‑making processes. Taken together, these insights underscore that institutional divergence operates on two interconnected planes: externally, in the evolving relationship between the UK and the EU, and internally, in the differentiated trajectories of Northern Ireland and Great Britain. This dual dynamic provides a crucial foundation for analyzing how Brexit reshapes governance both across borders and within the UK’s constitutional framework.

In Northern Ireland, institutional divergence becomes even more complex because it unfolds within a sensitive constitutional and peace-building environment. Whitten (2024) highlights Northern Ireland as a distinctive case within the UK internal market: post‑Brexit arrangements granted it a unique institutional position relative to Great Britain. Although the UK formally exited the EU, Northern Ireland continues to align with certain EU rules to avoid a hard border on the island of Ireland. Kelly and Tannam (2023) argue that Brexit prompted a shift in UK policy towards Northern Ireland, marked by unilateralism and a sovereignty‑driven approach. This recalibration has weakened traditions of cooperation associated with the peace process, demonstrating that institutional divergence is not merely a technical or legal phenomenon. It directly affects trust, identity, intergovernmental relations, and the stability of devolved governance. As a result, divergence generates tangible policy challenges—undermining coherence, heightening uncertainty, and complicating coordination between Northern Ireland and Great Britain. Policy evolution refers to the transformation of public policies, regulatory standards, and governance practices in response to institutional restructuring. Wolff and Piquet (2022) emphasize that post‑Brexit policy change should be understood as a trajectory rather than a single rupture, with British policies moving along paths of de‑Europeanisation, partial disengagement, renewed cooperation, or continued engagement depending on sectoral demands and political choices. Egan and Webber (2023) illustrate this dynamic in trade policy, noting the profound shifts that followed decades of collective EU agreements and regulatory alignment. Howell (2020) adds that policy evolution often takes the form of cautious regulatory adaptation, balancing the pursuit of autonomy with the practical need to preserve market access, regulatory equivalence, and legal certainty. Institutional divergence establishes the structural conditions that compel such adaptations. As institutions move away from common EU rules, governments must redesign legal frameworks, renegotiate responsibilities, adjust trade arrangements, and manage new regulatory boundaries. In this sense, institutional divergence functions as a key driver of observable policy change—shaping rules, regulatory alignments, governance negotiations, and sector‑specific adjustments across the post‑Brexit landscape.

New Institutionalism, introduced by March and Olsen (1984) in The New Institutionalism: Organizational Factors in Political Life, provides a valuable theoretical lens for analyzing institutional divergence and policy evolution. The framework emphasizes that political outcomes are not determined solely by individual preferences, party competition, or rational calculation; rather, they are shaped by institutions, rules, routines, norms, historical legacies, and established procedures that constrain and guide political behavior. Institutions possess relative autonomy and exert influence over the meaning, direction, and consequences of political action (March & Olsen, 1984). This perspective is particularly apt for studying the impact of Brexit on regulatory alignment, governance structures, legal arrangements, and policy coordination among Northern Ireland, Great Britain, and the EU. Scholars such as Hall and Taylor (1996), Pierson (2000), and Thelen (1999) reinforce the view that institutions sustain policy continuity and change through path‑dependent processes and gradual adaptation. Yet critics, including Hay and Wincott (1998), Peters et al. (2005), and Schmidt (2010), caution that institutionalist approaches may understate the role of agency, political conflict, and ideas. This critique is especially relevant in post‑Brexit Northern Ireland, where formal rules alone cannot explain divergence. Political contestation, unionist resistance, sovereignty claims, identity politics, and competing policy narratives surrounding the Protocol and Windsor Framework all demonstrate that institutional divergence is simultaneously structural and deeply political.

The reviewed studies predominantly employ qualitative, legal, doctrinal, historical, and policy‑analysis approaches. Birrell and Gray (2017) adopt qualitative policy‑review methods, examining constitutional arrangements, devolution debates, and EU‑related responsibilities in Scotland, Wales, and Northern Ireland. Howell (2020) applies doctrinal legal and regulatory analysis to assess UK and EU fund regulation, equivalence requirements, and the scope of regulatory autonomy. Egan and Webber (2023) use political economy and role theory frameworks to trace post-referendum trade debates and policy developments. Kelly and Tannam (2023) combine historical and policy analysis to explore the governance implications of the Protocol, while Whitten (2024) focuses on legal and policy review of the UK Internal Market Act, the Protocol, and the Windsor Framework. Birrell and Gray (2017) found Brexit had unequal impacts on devolved nations due to differing reliance on EU funding, legal frameworks, and political relationships, intensifying territorial and institutional tensions within the UK. Egan and Webber (2023) reported major changes in trade policy but noted that the “Global Britain” agenda faced constraints from post-Brexit EU relations and global political shifts. Kelly and Tannam (2023) observed weakened traditions of interdependence and cooperation, while Whitten (2024) highlighted Northern Ireland’s unique position, exposing the limits of a uniform internal market. Overall, institutional divergence has driven policy evolution, though outcomes remain contested, uneven, and dependent on effective coordination.

The impact of institutional divergence on policy evolution is neither automatic nor uniform; it varies according to prior Europeanisation, the sensitivity of policy domains, the strength of devolved institutions, and the willingness of governments to coordinate. Birrell and Gray (2017) highlight the need for stronger cooperation between Westminster and devolved administrations, recognition of devolved competences, and inclusive policy negotiation. Howell (2020) underscores the importance of carefully managed divergence in sectors where market access and regulatory credibility are critical. Whitten (2024) emphasizes transparent coordination and flexible cooperation among the UK, EU, and Northern Ireland. Collectively, these findings demonstrate that post‑Brexit governance extends well beyond legal separation from the EU. Effective management requires practical mechanisms to address regulatory differences, trade flows, environmental responsibilities, and constitutional stability. Institutional coordination, selective alignment, and cooperative governance emerge as essential strategies for mitigating the negative effects of divergence and fostering coherent policy evolution across the UK’s multi‑level governance system.

The existing literature reveals notable gaps in conceptual, theoretical, methodological, and empirical domains. Conceptually, key ideas such as institutional divergence and policy evolution are often examined in isolation or within narrow sectoral contexts rather than as interconnected variables. For instance, Howell (2020) focuses on financial regulation, Egan and Webber (2023) on trade policy, Birrell and Gray (2017) on devolution, Kelly and Tannam (2023) on political trust and sovereignty, and Whitten (2024) on Northern Ireland’s position in the internal market. Yet no comprehensive framework explains how divergence systematically drives policy evolution across regulatory, trade, environmental, constitutional, and governance dimensions. Theoretically, contemporary political approaches remain insufficiently integrated into a unified explanation of these processes. This study addresses this gap by conceptualizing institutional divergence as the independent variable and policy evolution as the dependent variable, applying New Institutionalism to explain how divergence in legal alignment, regulatory standards, intergovernmental relations, institutional authority, political agency, sovereignty claims, and governance contestation produces observable changes in policy coherence, trade arrangements, regulatory adaptation, and coordination. Methodologically, most existing studies rely on qualitative, doctrinal, historical, or legal-policy analysis without employing structured comparative frameworks capable of measuring the effects of divergence across post-Brexit policy areas. Empirically, research remains fragmented: Birrell and Gray (2017) on social policy, Howell (2020) on financial regulation, Egan and Webber (2023) on trade adaptation, Kelly and Tannam (2023) on political behaviour, and Whitten (2024) on Northern Ireland’s constitutional position. The current study addresses these limitations by adopting a comparative analytical approach that links divergence to observable policy outcomes across multiple domains—including regulatory alignment, legislative changes, negotiated frameworks, trade adjustments, environmental governance, policy coherence, and multi‑level coordination across Northern Ireland, Great Britain, and the wider United Kingdom.

This study adopts a comparative qualitative design to examine how institutional divergence has shaped policy evolution in the post‑Brexit United Kingdom, with particular focus on Northern Ireland. A comparative approach enables systematic analysis of Northern Ireland’s institutional position relative to Great Britain and the wider UK, highlighting similarities and differences in policy development across trade regulation, environmental policy, internal market arrangements, intergovernmental relations, and the Protocol/Windsor Framework. Institutional divergence is treated as the independent variable and policy evolution as the dependent variable, linking structural changes to observable outcomes. This framework provides a context-sensitive understanding of post-Brexit governance, moving beyond descriptive or sector-specific accounts and addressing methodological gaps in existing scholarship. It demonstrates how divergence in legal alignment, regulatory standards, and governance practices translates into concrete outcomes in policy coherence, trade, environmental regulation, and multi‑level coordination.

This study employs a documentary method consistent with its comparative qualitative design. Instead of primary data such as interviews or questionnaires, it draws on legal, institutional, policy, and scholarly documents that illuminate institutional divergence and policy evolution in the post‑Brexit UK and Northern Ireland. A purposive sampling strategy identified 30 key materials. Primary sources include the Protocol on Ireland/Northern Ireland, the Windsor Framework, the UK Internal Market Act, the UK–EU Trade and Cooperation Agreement, parliamentary reports, government papers, and Northern Ireland Assembly documents. Secondary sources comprise peer‑reviewed articles, books, legal analyses, and policy studies on Brexit, devolution, and governance. Documents were screened for relevance, credibility, authority, and connection to study variables. Key information was extracted using a structured review guide and comparative policy matrix, then organized thematically around regulatory divergence, legislative changes, policy coherence, trade, environmental governance, intergovernmental relations, and coordination. Comparative analysis contrasted Northern Ireland with Great Britain and the wider UK to identify how divergence has differently shaped policy evolution across jurisdictions.

Theoretical Perspective

This study is grounded in New Institutionalism, introduced by March and Olsen (1984), which emphasizes that political outcomes are shaped not only by interests or rational choice but also by institutional structures, norms, routines, and historical legacies. Institutions possess relative autonomy, influencing how actors define problems, interests, and responses. This makes the framework particularly suitable for examining how Brexit reshaped institutional arrangements, regulatory alignment, and policy coordination between the UK, Northern Ireland, and the EU, especially through the Protocol and Windsor Framework. Supportive scholarship highlights institutions as drivers of continuity and gradual change. Hall and Taylor (1996) stress historical, rational-choice, and sociological dimensions; Pierson (2000) explains path dependency; and Thelen (1999) shows how institutions evolve through adaptation and conflict. These perspectives underscore that Brexit was not a sudden rupture but an ongoing institutional process reshaping trade, regulation, and devolved governance. Brexit‑specific studies reinforce this lens. Birrell and Gray (2017) show varied devolved experiences; Wolff and Piquet (2022) frame Brexit as a continuum of de‑Europeanisation and re‑engagement; and Whitten (2024) highlights Northern Ireland’s exceptional position under EU rules. Together, they demonstrate that post‑Brexit policy evolution is deeply conditioned by institutional location and governance interactions. Critics caution against overemphasizing institutions while neglecting agencies and ideas. Hay and Wincott (1998) warn of exaggerated structural constraints; Peters et al. (2005) highlight the role of political conflict; and Schmidt (2010) advocated discursive institutionalism to capture the influence of ideas and narratives. These critiques are pertinent in Northern Ireland, where unionist resistance, sovereignty debates, and identity politics shape divergence alongside formal rules.

Despite these limitations, New Institutionalism remains the most appropriate framework. By treating institutional divergence as the independent variable and policy evolution as the dependent variable, the study demonstrates how changes in rules, authority structures, and norms drive outcomes across trade, environmental governance, internal market coordination, and intergovernmental relations in the UK’s post-Brexit landscape.

Institutional Divergence and Policy Coherence in Post-Brexit Governance

The study finds strong evidence that Brexit created significant territorial and institutional differences between Northern Ireland and Great Britain. While the United Kingdom as a whole voted to leave the European Union, Northern Ireland voted to remain, producing political and constitutional tensions. A uniform post-Brexit governance model could not easily accommodate Northern Ireland’s geographical position, peace process obligations, and economic ties to both the Republic of Ireland and the wider European market.

Referendum data empirically underpins the study’s argument. While the UK‑wide vote produced a Leave outcome, Northern Ireland voted Remain, highlighting internal political and institutional divergence. This contrast explains why Northern Ireland became a special case in the post‑Brexit settlement: its political preference, geographical border with the Republic of Ireland, and peace‑process obligations made a uniform UK‑wide withdrawal arrangement difficult to sustain (Electoral Commission, 2019; Northern Ireland Assembly, n.d.).

Table 1: Empirical Indicators for Verifying Institutional Divergence

 and Policy Evolution

Source: Electoral Commission (2019); Northern Ireland Assembly (n.d.); House of Lords Northern Ireland Scrutiny Committee (2026); Department for Business and Trade (2026a, 2026b); NISRA/Invest NI (2026).

Table 2: Brexit Referendum Results as Evidence of Territorial 

Political Divergence

Source: Electoral Commission (2019).

Tables 1 and 2 show that institutional divergence began with the different Brexit mandates and later became embedded in legal and regulatory arrangements. Northern Ireland’s partial alignment with EU single‑market laws for goods, unlike Great Britain, demonstrates that Brexit produced uneven institutional outcomes within the UK. This divergence directly affected policy coherence, as the UK sought sovereignty and regulatory autonomy while maintaining special arrangements for Northern Ireland. Consequently, post‑Brexit policy evolution became less about implementing a single national withdrawal and more about managing internal divergence.

Institutional Divergence Measures and the Management of Trae, Environmental, and 

Governance Challenges

The study shows that the Protocol, the Windsor Framework, the Internal Market Scheme, the green lane/red lane system, and the democratic consent mechanism operate as institutional measures to manage divergence. The strongest evidence relates to trade and governance, while environmental concerns are indirectly tied to regulatory alignment and EU single‑market rules, which embed standards affecting goods, compliance, and safeguards. Trade data confirms Northern Ireland’s economic position between Great Britain, the Republic of Ireland, and wider European markets. In 2024, sales to Great Britain totaled £20.1bn, while exports to the Republic of Ireland alone reached £10.45bn. These figures support the argument that Northern Ireland requires a distinctive governance arrangement, as its trade flows span both the UK internal market and the EU single market. The Windsor Framework and its green lane/red lane system exemplify policy evolution designed to manage the practical consequences of institutional divergence (Northern Ireland Assembly, n.d.; NISRA/Invest NI, 2026).

Table 3: Northern Ireland Trade Data as Evidence of 

Post-Brexit Policy Sensitivity

Source: NISRA/Invest NI (2026).

Table 3 illustrates why institutional divergence measures became necessary in Northern Ireland. Its economy is closely tied not only to Great Britain but also to the Republic of Ireland and wider European markets, making trade governance highly sensitive. A rigid UK-wide post-Brexit model risked increasing friction, undermining cross-border activity, and complicating governance on the island of Ireland. Distinctive arrangements under the Protocol, later refined through the Windsor Framework, therefore function as policy responses to these structural economic realities.

Policy Evolution through Legal Reform, Trade Adjustment, and Democratic Consent

The study shows that post‑Brexit governance has evolved through legal modification, differentiated trade mechanisms, and enhanced democratic scrutiny. Northern Ireland’s continued alignment with EU single‑market rules for goods confirms institutional divergence from Great Britain. At the same time, the Windsor Framework illustrates policy evolution, modifying the earlier Protocol through the Internal Market Scheme, red lane arrangements, labeling requirements, new scrutiny procedures, and the Stormont Brake.

Table 4: Legal and Governance Evidence of Policy Evolution

Sources: Northern Ireland Assembly (n.d.), House of Lords Northern Ireland Scrutiny Committee (2026), Department for Business and Trade (2026).

Table 4 confirms that post-Brexit policy evolution has unfolded through legal modifications, trade policy redesign, and new democratic scrutiny mechanisms. Northern Ireland’s continued alignment with EU single‑market rules for goods demonstrates institutional divergence from Great Britain. At the same time, the Windsor Framework illustrates policy evolution, revising the earlier Protocol through the Internal Market Scheme, red lane arrangements, labeling requirements, new scrutiny procedures, and Stormont Brake. The 2024 consent vote further shows that policy evolution is not merely administrative but also political, since the continuation of Articles 5–10 depends on institutional approval within Northern Ireland (House of Lords Northern Ireland Scrutiny Committee, 2026; Northern Ireland Assembly, n.d.).

The green and red lane arrangements distinguish goods remaining in Northern Ireland from those at risk of entering the EU single market, reducing trade burdens while preserving EU safeguards. Similarly, the democratic consent mechanism provides a political review process through which Northern Ireland’s institutions can periodically approve or contest key post‑Brexit arrangements. Together, these measures show that divergence has generated not only policy challenges but also new institutional tools for managing them.

Empirical evidence supports the study’s core argument. Brexit produced institutional divergence by separating the UK from the EU legal order while leaving Northern Ireland in partial regulatory alignment with the EU. At the same time, policy evolution is evident in the redesign of legal, trade, and governance mechanisms through the Protocol, the Windsor Framework, assimilated law reform, and democratic consent arrangements. Overall, the data confirm that institutional divergence has directly shaped policy evolution in post‑Brexit Northern Ireland and the wider United Kingdom.

Discussion of Major Findings

The findings align with Birrell and Gray (2017), who observed that Brexit produced uneven consequences across the devolved administrations, with Northern Ireland most affected due to its land border with the Republic of Ireland and its peace process context. This study similarly shows that Brexit deepened territorial, political, and institutional divergence within the UK. Northern Ireland’s Remain vote contrasted sharply with the wider UK's Leave outcome, placing it in a unique post-Brexit position where its regulatory and trade arrangements differ markedly from those of Great Britain. These results reinforce Birrell and Gray’s claim that Brexit intensified institutional tensions and exposed weaknesses in intergovernmental policy coordination.

This study supports the arguments of Howell (2020), Egan and Webber (2023), and Whitten (2024), particularly that institutional divergence does not automatically yield full policy independence but instead initiates complex processes of adaptation. Howell (2020) warned that complete regulatory divergence could create market‑access uncertainty, while Egan and Webber (2023) noted that the UK’s “Global Britain” trade agenda remains constrained by ongoing EU ties. This study confirms these insights, showing that UK–EU trade dependence persists and that Northern Ireland’s links with Great Britain, the Republic of Ireland, and wider European markets necessitate hybrid governance. Likewise, Whitten (2024) identified Northern Ireland as an exceptional case within the UK internal market—a finding reinforced here by evidence that the Protocol and Windsor Framework established a distinct regulatory regime, particularly through EU-aligned goods rules and the green/red lane system. This study only partially aligns with Kelly and Tannam (2023), who emphasized unilateralism, muscular unionism, political mistrust, and identity‑based contestation as Northern Ireland’s main post‑Brexit challenges. It concurs insofar as the 2024 democratic consent vote and lack of cross‑community support show that policy evolution remains politically contested. However, this research extends beyond their predominantly political interpretation by demonstrating that institutional divergence also produces tangible policy outcomes, shaping trade governance, regulatory alignment, legal reform, consent mechanisms, and multi‑level coordination. Thus, while Kelly and Tannam’s findings highlight instability and contestation, the present study offers a more comprehensive explanation by linking these tensions directly to policy evolution across legal, trade, environmental, and governance dimensions.

Conclusion

The study highlights institutional divergence as a defining feature of post‑Brexit governance in the United Kingdom. Brexit not only withdrew the UK from the EU but also created uneven institutional trajectories within the state. Northern Ireland now occupies a unique position, maintaining partial alignment with EU single‑market rules for goods, while Great Britain follows a separate regulatory path. This demonstrates that post-Brexit policy evolution is driven by institutional restructuring, regulatory adjustments, political negotiations, and ongoing UK–EU interdependence. The resulting divergence limits the feasibility of a uniform UK‑wide framework, as Northern Ireland’s regulatory position under the Protocol and Windsor Framework introduces legal complexity, political friction, and policy challenges. Future policy in Northern Ireland must prioritize flexible regulatory coordination over rigid assertions of sovereignty. Effective management will require sustained negotiation among Westminster, Northern Ireland’s institutions, the Irish government, and the EU to safeguard trade, uphold peace‑process commitments, and ensure policy coherence. Given Northern Ireland’s economic ties to Great Britain, the Republic of Ireland, and wider European markets, trade and market access remain central. The Windsor Framework, with its green and red lane system, illustrates how policy can reduce friction while respecting EU single‑market rules, underscoring the need to balance autonomy with interdependence. Policy evolution in Northern Ireland is as much political as administrative. Democratic consent mechanisms, debates over Articles 5–10 of the Windsor Framework, EU‑derived law reforms, and regulatory standards show that institutional divergence requires ongoing review, adaptation, and legitimacy. Managed well, divergence can foster innovation; mismanaged, it risks uncertainty, contestation, and fragmentation. Ultimately, successful post‑Brexit governance in Northern Ireland and the wider UK will depend on transparent legal frameworks, inclusive intergovernmental collaboration, and a realistic balance of sovereignty, stability, and shared governance.

We also suggest that a joint regulatory monitoring committee should be established to manage post-Brexit divergence between Northern Ireland and Great Britain. Its role would be to identify areas where Northern Ireland’s alignment with EU single-market rules diverges from UK regulations; review trade rules, environmental standards, legislative amendments, and consent mechanisms; and publish periodic reports on affected sectors. The committee should also recommend corrective measures to prevent regulatory gaps from escalating into political or economic problems. This is essential given Brexit’s regulatory fragmentation, overlapping legal frameworks, and policy uncertainty, particularly under the Protocol and Windsor Framework. Policy changes affecting trade, governance, environmental regulation, or internal market arrangements should involve consultation with political parties, business organizations, civil society, regulators, border communities, and UK–EU representatives. Each reform must include a clear impact assessment detailing effects on market access, cross‑border cooperation, political stability, regulatory certainty, and environmental protection. Such consultation reduces mistrust, strengthens democratic legitimacy, and ensures policy evolution addresses both sovereignty concerns and the practical needs of Northern Ireland’s economy, institutions, and communities. The study supports this recommendation, emphasizing the role of the Windsor Framework, green/red lane arrangements, assimilated law reforms, and democratic consent mechanisms in shaping post‑Brexit governance.

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