Many people confuse the UK with Great Britain or the British Isles.
So, let's see the differences!.
The United Kingdom (UK)
The United Kingdom of Great Britain and Northern Ireland (to give its full name) refers to the political union between England, Wales, Scotland and Northern Ireland. The UK is a sovereign state, but the nations that make it up are also countries in their own right.
This is their flag:
Great Britain
Great Britain is the official collective name of of England, Scotland and Wales and their associated islands. It does not include Northern Ireland and therefore should never be used interchangeably with ‘UK’!
Technically, if you lose the "Great", Britain only refers to England and Wales. Tricky!.
British Isles
This is only a geographical term. It refers to the islands of Great Britain and Ireland (including the Republic of Ireland) and the 5000 or so smaller islands in their coasts. Remember this only refers to geography, not nationality, and while the Republic of Ireland is part of the British Isles, its people are not British. A very important distinction!.
I hope that’s explained the very different meanings between UK, Great Britain and British Isles and why it’s important to use the right name at the right time. Geography really does matter!.
Source: "Ordnance Survey"