The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) and colloquially Great Britain (GB) or simply Britain, is a sovereign country in western Europe. 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 statethe 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 , the United Kingdom is the 78th-largest sovereign state in the world and the 11th-largest in Europe. It is also the 21st-most populous country, with an estimated 65.1 million inhabitants. Together, this makes it the fourth-most densely populated country in the European Union (EU). 

The United Kingdom is a constitutional monarchy with a parliamentary democracy. The monarch is Queen Elizabeth II, who has reigned since 6 February 1952. The capital of the United Kingdom and its largest city is London, a global city and financial centre with an urban area population of 10.3 million, the fourth-largest in Europe and second-largest in the European Union. Other major urban areas in the United Kingdom include the conurbations centred on Birmingham, Leeds, Glasgow, Liverpool and Manchester. The United Kingdom consists of four countries—England, Scotland, Wales and Northern Ireland. The last three have devolved administrations, each with varying powers, based in their capitals, Edinburgh, Cardiff and Belfast, respectively. The nearby Isle of Man, Bailiwick of Guernsey and Bailiwick of Jersey are not part of the United Kingdom, being Crown dependencies with the British Government responsible for defence and international representation.
Where in the world does it's coastline rank?
giving it the 12th-longest coastline in the world.