<Tr> <Th> ISO 3166 code </Th> <Td> GB </Td> </Tr> <Tr> <Th> Internet TLD </Th> <Td>. uk </Td> </Tr> <P> The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a sovereign country lying off the north - western coast of the European mainland . The United Kingdom includes the island of Great Britain, the north - eastern part of the island of Ireland and many smaller islands . Northern Ireland is the only part of the United Kingdom that shares a land border with another sovereign state‍--‌the Republic of Ireland . Apart from this land border, the United Kingdom is surrounded by the Atlantic Ocean, with the North Sea to its east, the English Channel to its south and the Celtic Sea to its south - south - west, giving it the 12th - longest coastline in the world . The Irish Sea lies between Great Britain and Ireland . With an area of 242,500 square kilometres (93,600 sq mi), the United Kingdom is the 78th - largest sovereign state in the world . It is also the 22nd-most populous country, with an estimated 66.0 million inhabitants in 2017 . </P> <P> The sovereign state is a constitutional monarchy with a parliamentary democracy . The monarch is Queen Elizabeth II, who has reigned since 1952, making her the longest - serving current head of state . The United Kingdom's capital and largest city is London, a global city and financial centre with an urban area population of 10.3 million . Other major urban areas in the UK include the conurbations centred on Manchester, Birmingham, Leeds, Glasgow and Liverpool . </P>

Which part of the world is the uk in