The inhabitants of Britain - the island containing England, Wales and Scotland - live in a state of some confusion over their group identity. Their cars, travelling abroad, display the letters GB (for Great Britain). Their diplomats, at international conferences, sit behind the letters UK (for United Kingdom).
Neither phrase is much used in ordinary conversation. The English, by far the majority within the United Kingdom, have a tendency to call their nation England - with notorious disregard for the sensibilities of the Welsh and the Scots, with whom they have been linked since 1536 and 1707 respectively.
The more widely acceptable name, also in common use, is Britain. Its prevalence is reflected in phrases such as the British empire (something which even the English have never claimed as their own) and in the colloquial modern term 'Brits' for inhabitants of the island.
Historically 'united kingdom' begins life in informal use during the 18th century to describe the newly combined nation of England and Scotland. It becomes official in 1800, in the Act of Union with Ireland, when the enlarged kingdom is called the United Kingdom of Great Britain and Ireland. The earlier Act of Union, of 1707, states merely that England and Scotland shall 'be united into one kingdom by the name of Great Britain'