The United Kingdom functions as a constitutional monarchy and parliamentary democracy under the Westminster system, also known as a democratic parliamentary monarchy. It is a centralised, unitary state, with sovereignty vested in the Parliament of the United Kingdom, which comprises the elected House of Commons, the appointed House of Lords, and the Crown, represented by the monarch. Legislation requires royal assent to become law, and the UK constitution is uncodified, made up of statutes, case law, international treaties, and constitutional conventions. Key principles, such as parliamentary sovereignty, the rule of law, democracy, and adherence to international law, are recognised by the Supreme Court.

King Charles III serves as the monarch and head of state of the UK and 14 other independent sovereign states, collectively known as the Commonwealth realms. While formally vested with executive authority, most royal powers, including the royal prerogative, are exercised on the advice of ministers responsible to Parliament. The monarch retains reserve powers to consult, encourage, and warn, helping safeguard responsible government and prevent constitutional crises.

The United Kingdom is divided into 650 parliamentary constituencies for general elections to the House of Commons, with each constituency represented by one member of Parliament (MP) elected through a first-past-the-post system. MPs serve terms of up to five years before seeking re-election. Since the 1920s, the Conservative Party and the Labour Party have dominated British politics, although smaller parties have occasionally gained seats in Parliament.

The prime minister serves as head of government, leading a Cabinet of senior ministers to implement public policy, administer services, and advise the monarch. The prime minister is conventionally an MP, leader of the party with the most Commons seats, and holds office by maintaining the confidence of the House of Commons. Additional roles traditionally held by the prime minister include First Lord of the Treasury, Minister for the Civil Service, and Minister for the Union. As of 2025, Sir Keir Starmer of the Labour Party holds the office of prime minister.

While not formally part of the UK, Crown Dependencies such as Jersey, Guernsey, and the Isle of Man, along with 14 British Overseas Territories, remain under the sovereignty of the British Crown, illustrating the continued global reach of the UK’s constitutional framework.