The largest island of the British Isles, with its adjacent island s, in NE Europe, it now forms part of the United Kingdom. The history of Great Britain is that of England , Wales, and Scotland . It was united under one king in 1603 when the Scottish and English Crowns were joined under James VI of Scotland . Great Britain became a single political unit in 1707, following the Act of Union. After the union of the British and Irish parliaments in 1801, Great Britain became part of the larger national unit of the United Kingdom of Great Britain and Northern Ireland .