Impact of Brexit on the UK’s Unitary Structure and Centralised Power

Brexit addresses trade and borders; the implications of this have changed the very organization and exercise of power in the UK. Britain has been a unitary country for many centuries. It is a unitary state in the sense that the legal sovereignty of that state resides with the center, which I take to be the UK Parliament at Westminster. Devolution has softened some of the harsher edges of centralization since the late 1990s, but did not replace it. Now Brexit knocked down that equilibrium, and apart from the bounded supranational framework, it raised the issues of parliamentary sovereignty, devolution, and territorial integrity again into discussion.

What “Unitary” Means in the UK

A unitary state centralizes constitutional authority with one sovereign legislature: 

Parliamentary sovereignty means that Parliament can make or unmake any law.

Devolved legislatures—the Scottish Parliament, Senedd Cymru (Welsh Parliament), and the Northern Ireland Assembly—exercise devolved powers: these are powers granted to those legislatures by an Act of Parliament and which can be changed by Westminster.

An entrenched, codified constitution does not prevent Westminster from changing the devolution settlement.

Before Brexit, legal constraints were placed on both the center and devolved institutions in the UK: when it comes to areas of EU competence, EU law has supremacy. After Brexit, that restriction of external influence is largely gone, reinstating the supremacy of the law of Westminster. This clear legal impact of “Brexit” made it possible for Westminster’s legal supremacy to be re-established, but the obligation to work together (politically) across the nations remained problematic.

Life Before Brexit: A Common European Framework

Before 2016, many policy areas, agriculture, fisheries, the environment, and food standards, were governed by EU law. Devolved governments implemented this European law within their own devolved responsibilities, while UK ministers represented the whole UK in the Brussels-based process. In this regard, the EU was very much treated as a neutral site for developing common EU standards and managing disputes:

1. It provided reduced friction between the center and the devolved governments, since common EU standards were coordinated at the EU level.
2. It ensured regulatory alignment across the UK internal market by default, since all parts of the UK were adopting the same EU obligations.
3. It contained conflict over sovereignty, since everyone was still subject to the same supranational framework for law and adjudication.

Brexit has eliminated supranational layer and resulted in the UK needing to create its own inducements (and constraints) for coordination across the four nations.

Parliamentary Sovereignty Re-Asserted

Brexit generated large-scale publicity cases, most notably Miller (2017) on the need for parliamentary authorization to trigger Article 50 and Miller/Cherry (2019), which considered the role of prorogation, restating important constitutional principles, changing the overall effect of centralization in Parliament. 

1. The European Union (Withdrawal) Act 2018 converted EU law into retained EU law, allowing Parliament (and, where relevant, devolved legislatures) to modify it. 
2. The override ability of Westminster developed further as the supremacy of EU law fell away.
3. The Retained EU Law (Revocation and Reform) Act 2023 extended ministerial discretion to amend or sunset inherited EU laws, another instance of executive-led centralization, which raised questions about scrutiny. 

In terms of legal theory, this took the UK closer to the purist unitary model. In this model, the UK Parliament and UK ministers are in a position to redirect regulatory choices post-EU membership. 

The United Kingdom Internal Market Act 2020 

The United Kingdom Internal Market Act (UKIMA) 2020 is a concrete embodiment of Brexit’s centralizing gravitational pull. UKIMA’s key concepts – mutual recognition and non-discrimination have been designed to allow goods and services to move freely in and out of England, Scotland, Wales, and Northern Ireland without the need for homogeneity, even if the same subject area is governed by different standards in each territory.

Why it matters for the unitary structure:

1. It limits the practical implications of devolved divergence. For example, if Scotland sets a higher product standard, it still must accept goods lawfully produced in England (mutual recognition).
2. Devolved governments have argued that this limits their policy autonomy because local regulations may be undermined by imports produced with lower standards within the UK.
3. The act enables the UK government to act as the system designer of the internal market, re-centralizing the rule-making role that the EU had previously provided.

It is necessary for economic coherence following Brexit, while critics see it as a re-centralization through structural reform that affects devolution by stealth.

Common Requirements: Cooperation or Control?

As a substitute for the coordinating function of the EU, the UK and devolved governments have developed Common Requirements, which are sector-specific agreements that cover policy areas, including agriculture, food labeling, chemicals, and the environment.

1. In principle, they allow for joint governance and sharing of information and a process for agreed divergence.
2. Because they are non-statutory and have limited public transparency, their effectiveness is reliant on political trust.
3. Where consensus is not built, there is the potential for the UKIMA market access principles to veto the practical impact of devolved rules.

Situational and political frameworks have made an effort to govern a multi-level state without a formal constitution. The ability for the UK to do this relies on good faith intergovernmental relations, something that has been under pressure as a result of Brexit since 2016.

Judicial Review, Rights, and Constitutional Norms

EU membership has embedded a rights culture framed by EU law and the Charter of Fundamental Rights (the Human Rights Act 1998 continues to be the main UK rights instrument, holding the European Convention on Human Rights). Regarding rights post-Brexit:

1. The removal of the EU Charter as well as limitations on the applicability of some EU general principles in domestic law lessens some rights-based processes and constraints on government.
2. The courts maintain the same levels of judicial review, but the interpretive space is in flux as retained EU law is revised or repealed. 
3. Debates continue regarding the extent to which the balance between the elected and legally supervised aspects of a unitary state should be amended.

Brexit is nudging the constitution further towards greater political control of legal enforcement unless checked by strong domestic rights and legislative scrutiny. 

Trade Agreements and External Relations

Treaty negotiations are a reserved power, but post-Brexit trade agreements impact devolved competence (e.g., agriculture, food, and environment):

1. The UK government conducts the negotiations, but shared powers and input are important for devolved administrations to influence agreements and expressly protect local interests, rather than accepting unbinding consensus documents.
2. When agreements necessitate internal implementation, UKIMA and common frameworks operate as mediating spaces to comply.
3. The tension shows a unitary state transforming to multi-tier policymaking and not yet having a fully codified expanse of external powers.

The UK is unitary for treaty purposes, yet internally plural when those treaties involve devolved areas.

Has Brexit made the UK more or less unitary?

Legally more unitary; politically more plural. That contradiction is at the heart of the shift.

1. Unitary in law: With the loss of EU supremacy, Westminster sits firmly at the apex. UKIMA 2020 and reforms to retain EU law consolidate central standard-setting and flexibility for the executive.
2. Plural in practice: The Windsor Framework institutionalizes asymmetry in Northern Ireland; devolved administrations pursue distinct social and environmental policies; common frameworks support the idea of negotiated divergence.

The UK’s constitutional order is now posing a more challenging question: How does a unitary state manage profound internal diversity without a codified constitution to arbitrate disagreement?