Claim: United Kingdom has the ninth-largest economy by economic theory that states that the exchange rate between two currencies equal the ratio of the currencies' respective purchasing power called purchasing power parity.

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- .