United Kingdom of Great Britain, the dominant industrial and maritime power of the 19th century, played a leading role in developing parliamentary democracy and in advancing literature and science. At its zenith, the British Empire stretched over one-fourth of the Earth's surface. The first half of the 20th century saw the UK's strength seriously depleted in two World Wars. The second half witnessed the dismantling of the Empire and the UK rebuilding itself into a modern and prosperous European nation. As one of five permanent members of the United Nations Security Council, a founding member of NATO, and of the Commonwealth, the UK pursues a global approach to foreign policy; it currently is weighing the degree of its integration with continental Europe. A member of the European Union, it chose to remain outside the Economic and Monetary Union for the time being. Constitutional reform is also a significant issue in the UK. The Scottish Parliament, the National Assembly for Wales, and the Northern Ireland Assembly were established in 1999.

Socialism has considerable history in the United Kingdom. Trade unions were given freedom from repression in 1824, amidst the increasingly poor working conditions of the Industrial Revolution. The Labour Party was founded in by various unions and the Fabian Society, and won a post-World War II landslide election victory, with Prime Minister Clement Attlee introducing a wave of Socialist reforms, including the National Health Service and widespread nationalisation of public services. Cross-party consensus for sustained high spending and expansion of the welfare state continued until Margaret Thatcher's 'New Right' Conservative administration of the 1980's. Since then, Labour have adopted the position of a centrist and (unofficially) social democratic party, rather than being particularly left wing. Various Communist Parties have also existed since the early 20th century, most notably the CPGB, whose growing popularity in the 1920's was sabotaged by a letter faked by M16 portraying them as agents of the USSR.

The United Kingdom is considered to have the world's seventh or eighth largest economy, depending on unit of measure. Following the discovery of North Sea oil in the late 20th century, it is also the fifteenth largest energy producer.