The United Kingdom is a constitutional monarchy and unitary state consisting of four countries: England, Northern Ireland, Scotland and Wales. It is governed by a parliamentary system with its seat of government in London, the capital, but with three devolved national administrations in Belfast, Cardiff and Edinburgh, the capitals of Northern Ireland, Wales and Scotland respectively. The Channel Island bailiwicks of Jersey and Guernsey, and the Isle of Man are Crown Dependencies and not part of the UK. The UK has fourteen overseas territories, all remnants of the British Empire, which at its height in 1922 encompassed almost a quarter of the world's land surface, the largest empire in history. British influence can continue to be observed in the language, culture and legal systems of many of its former colonies.

The UK is a developed country, with the world's sixth largest economy by nominal GDP and the seventh largest by purchasing power parity。 It was the world's first industrialized country and the world's foremost power during the 19th and early 20th centuries, but the economic cost of two world wars and the decline of its empire in the latter half of the 20th century diminished its leading role in global affairs. The UK nevertheless remains a major power with strong economic, cultural, military, scientific and political influence. It is a nuclear power and has the fourth highest defense spending in the world. It is a Member State of the European Union, holds a permanent seat on the United Nations Security Council, and is a member of the Commonwealth of Nations, G8, OECD, NATO, and the World Trade Organization.