British - choose a jigsaw puzzle to solve

The United Kingdom of Great Britain and Northern Ireland , commonly known as the United Kingdom (UK) or Britain, is a sovereign country in western Europe . Lying off the north-western coast of the European mainland, the UK 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 UK 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 UK is the 78th-largest sovereign state in the world . It is also the 21st-most populous country , with an estimated 65.5 million inhabitants in 2016 . The UK 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 UK's capital and largest city is London , a global city and financial centre with an urban area population of 10.3 million .