Claim: The Celtic Sea borders the UK.

The United Kingdom of Great Britain and Northern Ireland , commonly known as the United Kingdom -LRB- UK -RRB- or 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.Although Northern Ireland is the only part of the UK that shares a land border with another sovereign state , two of its Overseas Territories also share land borders with other sovereign countries . Gibraltar shares a border with Spain , while the Sovereign Base Areas of Akrotiri and Dhekelia share borders with the Republic of Cyprus , the Turkish Republic of Northern Cyprus and the UN buffer zone separating the two Cypriot polities . 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 242500 km2 , 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 -LRB- EU -RRB- .   The United Kingdom is a constitutional monarchy with a parliamentary system of governance . 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 .   The relationships among the countries of the UK have changed over time . Wales was annexed by the Kingdom of England under the Laws in Wales Acts 1535 and 1542 . A treaty between England and Scotland resulted in 1707 in a unified Kingdom of Great Britain , which merged in 1801 with the Kingdom of Ireland to form the United Kingdom of Great Britain and Ireland . Five-sixths of Ireland seceded from the UK in 1922 , leaving the present formulation of the United Kingdom of Great Britain and Northern Ireland.The Anglo-Irish Treaty was signed on 6 December 1921 to resolve the Irish War of Independence . When it took effect one year later , it established the Irish Free State as a separate dominion within the Commonwealth . The UK 's current name was adopted to reflect the change . There are fourteen British Overseas Territories . These are the remnants of the British Empire which , at its height in the 1920s , encompassed almost a quarter of the world 's land mass and was the largest empire in history . British influence can be observed in the language , culture and legal systems of many of its former colonies .   The United Kingdom is a developed country and has the world 's fifth-largest economy by nominal GDP and ninth-largest economy by purchasing power parity . The UK is considered to have a high-income economy and is categorised as very high in the Human Development Index , ranking 16th in the world . It was the world 's first industrialised country and the world 's foremost power during the 19th and early 20th centuries . The UK remains a great power with considerable economic , cultural , military , scientific and political influence internationally . It is a recognised nuclear weapons state and is seventh in military expenditure in the world . The UK has been a permanent member of the United Nations Security Council since its first session in 1946 . It has been a leading member state of the EU and its predecessor , the European Economic Community -LRB- EEC -RRB- , since 1973 . However , on 23 June 2016 , a national referendum on the UK 's membership of the EU resulted in a decision to leave , and its exit from the EU is currently being negotiated . The UK is also a member of the Commonwealth of Nations , the Council of Europe , the G7 finance ministers , the G7 forum , the G20 , NATO , the Organisation for Economic Co-operation and Development -LRB- OECD -RRB- , and the World Trade Organization -LRB- WTO -RRB- .