There are also 14 British Overseas Territories, the last remnants of the British Empire which, at its height in the 1920s, encompassed almost a quarter of the world's landmass and a third of the world's population, and was the largest empire in history. The nearby Isle of Man, Guernsey and Jersey are not part of the UK, being Crown Dependencies with the British Government responsible for defence and international representation. Most of Ireland seceded from the UK in 1922, leaving the present United Kingdom of Great Britain and Northern Ireland, which formally adopted that name in 1927. Its union in 1801 with the Kingdom of Ireland created the United Kingdom of Great Britain and Ireland.

The Treaty of Union between the Kingdom of England (which included Wales, annexed in 1542) and the Kingdom of Scotland in 1707 formed the Kingdom of Great Britain. The United Kingdom has evolved from a series of annexations, unions and separations of constituent countries over several hundred years. Other than England, the constituent countries have their own devolved governments, each with varying powers. The United Kingdom consists of four countries: England, Scotland, Wales and Northern Ireland. The capital and largest city is London, a global city and financial centre with a metropolitan area population of 14 million. The monarch, Queen Elizabeth II, has reigned since 1952. The United Kingdom is a unitary parliamentary democracy and constitutional monarchy. The total area of the United Kingdom is 242,500 square kilometres (93,628 sq mi), with an estimated population in 2020 of 68 million. The Irish Sea separates Great Britain and Ireland. Otherwise, the United Kingdom is surrounded by the Atlantic Ocean, with the North Sea to the east, the English Channel to the south and the Celtic Sea to the south-west, giving it the 12th-longest coastline in the world. Northern Ireland shares a land border with the Republic of Ireland. The United Kingdom includes the island of Great Britain, the north-eastern part of the island of Ireland, and many smaller islands within the British Isles.

The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom ( UK) or Britain, is a sovereign country in north-western Europe, off the north-western coast of the European mainland.

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