The UK constitution is unitary — sovereignty (or ultimate authority) has traditionally been located at the centre, with the component parts of the country — England, Scotland, Wales and Northern Ireland —all essentially run from London and treated in a similar way. This has been modified since the introduction of devolution in the late 1990s.

Some would now use the term 'union state' to describe the UK since, although the centre remains strong, the individual sub-national units are governed in different ways. The distribution of power between the central and regional governments of the UK can still be altered by an act of Parliament. This is an important difference with a federal constitution like that of Germany or the USA.

Constitutions have also been classified in terms of their content and, specifically, by the institution or structure they underpin. The most widely used such classification is between unitary and federal constitutions. Unitary constitutions establish the constitutional supremacy of central government over provincial or local bodies. They do this by vesting sovereignty in the national legislature, meaning that it can create or abolish, strengthen or weaken, all other institutions. In the UK, this is reflected in the fact that Parliament possesses, at least in theory, unrivalled and unchallengeable legislative authority. Devolved assemblies and local authorities do not, therefore, enjoy a share of sovereignty. By contrast, federal constitutions divide sovereignty between two levels of government. Both central government (the federal level) and regional government (the state level) possess a range of powers that the other cannot encroach on. Many argue that as the devolution has deepened the UK constitution has acquired a ‘quasi-federal’ character.