Comparing the UK and US Constitutions

The USA has one constitutional document. The UK has none. Every other difference in this comparison follows from that.

The US Constitution was drafted in 1787 and came into force in 1789. It is codified (contained in a single text), entrenched (protected by a special amendment procedure) and superior to ordinary law, so a statute that conflicts with it can be struck down by the courts. It is also famously short and often vague: phrases such as "necessary and proper" and "cruel and unusual" leave enormous interpretive room, which is why so much American constitutional argument happens in the Supreme Court rather than in Congress.

The UK constitution is uncodified, unentrenched and unitary. It rests on two pillars: parliamentary sovereignty — Parliament can make or unmake any law — and the rule of law. Because no rule is entrenched, a constitutional change and an ordinary policy change use the identical procedure.

	USA	UK
Form	Single codified document, 1787	Uncodified, drawn from several sources
Status	Superior to statute	Constitutional statutes rank alongside ordinary statutes
Amendment	Article V supermajorities	Ordinary Act of Parliament
Interpreter	Supreme Court, which may strike down law	Courts interpret; Parliament retains the last word
Territorial principle	Federal	Unitary, with devolution

Both constitutions draw on more than their headline source, and the overlap is greater than students usually expect.

The five sources of the UK constitution:

• Statute law — Acts of Parliament, from the Bill of Rights (1689) and the Acts of Union (1707) to the Parliament Acts (1911 and 1949), the Human Rights Act 1998 and the Constitutional Reform Act 2005.
• Common law — judge-made law and legal principle developed through decided cases.
• Conventions — binding customs with no legal force, such as the Salisbury Convention and the rule that the monarch appoints as Prime Minister whoever can command the confidence of the Commons.
• Authoritative works — texts such as Bagehot's The English Constitution (1867) and Erskine May, which describe practice rather than create it.
• Treaties — international agreements affecting the UK's constitutional position.

The sources of the US Constitution are narrower on paper and wider in practice: the original seven Articles, the 27 ratified amendments, the interpretations of the Supreme Court that give the text operative meaning, and a body of unwritten convention. The two-term presidential limit began as a convention set by George Washington, was broken by Franklin Roosevelt, and was only then written into the 22nd Amendment in 1951.

The instructive similarity is that both systems rely on convention and judicial interpretation alongside their formal texts. The difference is that the UK depends on convention for questions the USA settles in writing.

This is the sharpest structural contrast, and it produces a genuinely different politics.

Worked example — Article V. A US constitutional amendment must clear two stages:

1. Proposal — a two-thirds vote in both the House of Representatives and the Senate, or a convention called by two-thirds of state legislatures, a route not used to date.
2. Ratification — approval by three quarters of the states, currently 38 of 50.

The result is extreme rigidity. Thousands of amendments have been proposed since 1789 and 27 have been ratified. Ten of those — the Bill of Rights — arrived together in 1791, so only 17 have passed in the 235 years since. The most recent to take effect, the 27th, deals with congressional pay and was ratified in 1992. The Equal Rights Amendment, proposed by Congress in 1972, failed to reach 38 states within its ratification window.

Worked example — the UK equivalent. Changing the UK constitution requires a majority in the Commons, the consent or exhaustion of the Lords, and Royal Assent. The Labour government elected in 1997 used that procedure to create devolved legislatures in Scotland, Wales and Northern Ireland, incorporate the European Convention on Human Rights through the Human Rights Act 1998, remove most hereditary peers, and — through the Constitutional Reform Act 2005 — create a UK Supreme Court that opened in 2009. Any of those could be reversed by a later Act passed on a simple majority.

	Amending the US Constitution	Amending the UK constitution
Threshold	Two-thirds of Congress plus three quarters of states	Simple Commons majority
Actors with a veto	Either chamber, or 13 states	In practice, none once the Commons has decided
Amendments achieved	27 since 1789	Continuous change, several major reforms since 1997
Risk	Rules that cannot adapt to modern conditions	Rules a temporary majority can rewrite

The US Constitution assigns the three functions of government to three branches with distinct powers, distinct personnel and distinct buildings. Article I vests legislative power in Congress, Article II executive power in the President, Article III judicial power in the Supreme Court. Membership does not overlap: a member of Congress appointed to the Cabinet must resign their seat.

Checks and balances then require the branches to cooperate:

Branch	Checks it holds
Congress on the President	Reject legislation, override a veto with two-thirds of both chambers, confirm or refuse appointments, ratify treaties by two-thirds of the Senate, control appropriations, impeach and try
President on Congress	Veto bills, call special sessions, shape the agenda through the State of the Union
Supreme Court on both	Judicial review of the constitutionality of federal and state action
Congress and President on the Court	Nomination and Senate confirmation of Justices; impeachment; proposing amendments

The UK fuses executive and legislature. Ministers are drawn from and sit in Parliament, and the government exists only while it holds the confidence of the Commons. That fusion is what produces Lord Hailsham's charge of an "elective dictatorship": a government with a secure majority controls the legislature that is supposed to check it. Labour's majority of 174 in 2024, won on 33.7% of the UK vote, is the standard illustration.

Partial separation does exist in the UK. The Constitutional Reform Act 2005 removed the Law Lords from the House of Lords and reduced the Lord Chancellor's judicial role, giving the judiciary institutional independence it previously lacked.

Both systems distribute power territorially, and the specification asks you to compare them directly. The distinction is about where sovereignty sits, not about how much power the sub-national units actually exercise.

US federalism is constitutional. The federal government holds enumerated powers; the Tenth Amendment reserves everything else to the states or the people. States have their own constitutions, legislatures, governors, courts and criminal codes. Congress cannot abolish a state or unilaterally rewrite the division of powers, because that division sits in the Constitution itself. Where the boundary lies remains contested: abortion law reverted to the states after the Supreme Court overturned Roe v Wade in 2022, while the federal government's reach through funding conditions and commerce regulation has grown for a century.

UK devolution is statutory. The Scotland Act 1998, the Government of Wales Act 1998 and the Northern Ireland Act 1998 transferred powers to bodies Westminster created and Westminster can, in law, alter. Two pieces of evidence show that the difference is real rather than theoretical:

• The Northern Ireland Assembly has been suspended more than once. Devolved government was restored on 3 February 2024, with Michelle O'Neill (Sinn Féin) as First Minister and Emma Little-Pengelly (DUP) as deputy First Minister.
• The Senedd was reshaped wholesale for the election of 7 May 2026, expanding from 60 to 96 members returned from 16 six-member constituencies under a closed proportional list system. A change on that scale to a US state legislature would require action by that state, not by Washington.

Devolution has nonetheless become politically difficult to reverse. Referendums approved it, and the May 2026 Senedd election delivered a result — Plaid Cymru 43 seats, Reform UK 34, Labour 9 — that no Westminster government could set aside without a constitutional crisis. Legal sovereignty and political sovereignty have parted company.

The specification requires you to account for these similarities and differences using the rational, cultural and structural approaches.

Structural. The clearest and strongest explanation of most constitutional differences. The USA has an entrenched codified document because the Founders designed one; the amendment threshold in Article V then locks in that design regardless of what any generation of politicians wants. The UK has a flexible constitution because parliamentary sovereignty makes entrenchment logically impossible: a Parliament that could bind its successors would not be sovereign. Structure also explains an important similarity: both systems have developed a powerful final court, because any system that places legal limits on executive action needs an institution to police them.

Cultural. Structure cannot explain why Americans revere their Constitution while Britons argue about theirs as ordinary politics. The 1787 document functions as a symbol of national identity, defended by groups across the political spectrum, which raises the political cost of amendment far above the legal cost. British political culture has treated constitutional arrangements as practical machinery since at least 1997, which is why a government could rewire the judiciary, the second chamber and the territorial state within a decade and face no comparable backlash.

Rational. Individual actors defend the constitutional rules that benefit them. Smaller US states support the Article V ratification threshold and the equal representation of states in the Senate because those rules magnify their influence. UK governments resist codification because an uncodified constitution leaves the executive with more freedom of action, and the party that wins under existing arrangements has little incentive to change them. The rational approach also explains constitutional change that culture and structure cannot: devolution in 1997 served Labour's calculation about the electoral threat from the SNP as well as any principle.

A comparative answer that applies one approach to a difference and a different approach to a similarity shows more analytical range than one that runs all three approaches over the same point.

1. Saying the UK has "no constitution"

The UK constitution is uncodified, not non-existent. Statute, common law, conventions, authoritative works and treaties all supply constitutional rules. Getting this wrong undermines everything that follows.

2. Treating devolution and federalism as synonyms

The comparison is about the location of sovereignty. States are constitutionally guaranteed; devolved bodies are statutory creations. Say that, then concede the political reality that devolution has become entrenched in practice.

3. Assuming a codified constitution is automatically better at protecting rights

Entrenchment protects rights against a temporary majority, but it also freezes eighteenth-century language and hands enormous power to nine unelected Justices. Set out the trade-off rather than asserting one side of it.

4. Listing differences and skipping similarities

Both constitutions rely on convention, both have been reshaped by judicial interpretation, both distribute power territorially, and both place limits on executive action. Similarities are as creditable as differences.

5. Applying the approaches as a bolted-on paragraph

Sections 6.2.1 and 6.2.2 are assessed together. Weave the approach into the comparison — "the structural reason for this is…" — rather than appending a block of theory at the end.

6. Using stale constitutional examples

Devolution has moved. The Senedd's new 96-member closed-list system was used for the first time on 7 May 2026, and the Northern Ireland Executive has been sitting since 3 February 2024. Date every example you use.