The United Kingdom is a constitutional monarchy which, by legislation and convention, operates as a unitary parliamentary democracy. A hereditary monarch, currently King Charles III, serves as head of state while the prime minister of the United Kingdom, currently Sir Keir Starmer since 2024, serves as the head of the elected government.

Under the United Kingdom's parliamentary system, executive power is exercised by His Majesty's Government, whose prime minister is formally appointed by the king to act in his name. The king must appoint a member of parliament that can command the confidence of the House of Commons, usually the leader of the majority party or apparent majority party, though the king may choose to appoint an alternative if they say that they cannot expect the confidence of the House. Having taken office, the prime minister can then appoint all other ministers from parliament.

The Parliament has two houses: the House of Commons and the House of Lords. The Crown in Parliament is the UK's supreme legislative body, with unlimited powers of legislation subject only to convention. Normally bills passed by both Houses become law when presented for Royal Assent. However, there is provision in the Parliament Acts by which the democratically elected House of Commons could exceptionally obtain Royal Assent to a bill which the House of Lords has repeatedly failed or refused to pass. However, any use of this Parliament Acts procedure could provoke a constitutional crisis.

Parliament has devolved some legislative powers to the parliaments of Scotland and Wales and the assembly of Northern Ireland. Many other limited powers are granted by statute to the Privy Council, H.M.Ministers or other authorities, to make delegated legislation on particular subjects.

The British political system is a multiple-party system and was according to the V-Dem Democracy Indices 2023 the 22nd most electorally democratic in the world. From the 1920s to date, the two dominant parties have been the Conservative Party and the Labour Party. Before the Labour Party rose in British politics, the Liberal Party was the other major political party, along with the Conservatives. While coalition and minority governments have been an occasional feature of parliamentary politics, the first-past-the-post electoral system used for general elections tends to maintain the dominance of these two parties, though each has in the past century relied upon a third party, such as the Liberal Democrats, to deliver a working majority in Parliament. A Conservative–Liberal Democrat coalition government held office from 2010 until 2015, the first coalition since 1945. The coalition ended following parliamentary elections on 7 May 2015, in which the Conservative Party won an outright majority of seats, 330 of the 650 seats in the House of Commons, while their coalition partners lost all but eight seats.

With the partition of Ireland, Northern Ireland received home rule in 1920, though civil unrest meant direct rule was restored in 1972. Support for nationalist parties in Scotland and Wales led to proposals for devolution in the 1970s, though only in the 1990s did devolution happen. Today, Scotland, Wales and Northern Ireland each possess a parliament/assembly and a government, with devolution in Northern Ireland being conditional on participation in certain all-Ireland institutions. The British government remains responsible for non-devolved matters and, in the case of Northern Ireland, co-operates with the government of the Republic of Ireland. Devolution of executive and legislative powers may have contributed to increased support for independence in the constituent parts of the United Kingdom. The principal Scottish pro-independence party, the Scottish National Party, became a minority government in 2007 and then went on to win an overall majority of MSPs at the 2011 Scottish Parliament elections which formed the current Scottish Government administration. In a 2014 referendum on independence 44.7% of voters voted for independence versus 55.3% against. In Northern Ireland, Irish nationalist parties such as Sinn Féin advocate Irish reunification. In Wales, Welsh nationalist parties such as Plaid Cymru support Welsh independence.

The constitution of the United Kingdom is uncodified, being made up of constitutional conventions, statutes and other elements. This system of government, known as the Westminster system, has been adopted by other countries, especially those that were formerly parts of the British Empire. The United Kingdom is also responsible for several other territories, which fall into two categories: the Crown Dependencies, in the immediate vicinity of the UK, are strictly-speaking subject to the British Crown (i.e., the Monarch) but not part of the United Kingdom (though de facto British territory), and British Overseas Territories, as British colonies were re-designated in 1983, which are part of the sovereign territory of the United Kingdom, in which different aspects of internal governance have been delegated to local governments, with each territory having its own first minister, (though the titles differ, such as in the case of the Chief Minister of Gibraltar). They remain subject to the Parliament of the United Kingdom (which refers only to Great Britain and Northern Ireland, governed directly by the British Government, and not via local subsidiary governments or officers.

The British monarch, currently King Charles III, is the head of state of the United Kingdom. Though he takes little direct part in government, the Crown remains the fount in which ultimate power over the executive government, the judiciary, the legislature and the established Church of England formally lies. These powers are known as the royal prerogative and cover a vast amount of things, such as the issue or withdrawal of passports, the appointment or dismissal of the prime minister or even the declaration of war. The powers are delegated from the Crown primarily to the prime minister, who may freely appoint privy councillors, junior ministers and other officers and servants of the Crown, (civil, military, diplomatic, and others such as the secret services) to exercise them. All powers are subject to the Rule of Law, so that the legality of their exercise may always be judicially reviewed and quashed by the High Court, and their exercise is supervised by Parliament when it is sitting. The exercise of most powers by His Majesty's Government generally does not require the consent of Parliament; but certain statutory powers are subject to positive or negative resolutions of Parliament, notably the powers to make delegated legislation by Statutory Instruments, and to make certain Rules and Orders.