London is what to Great Britain?

Answer 1

London is the capital of England and of the United Kingdom of Great Britain and Northern Ireland.

Great Britain consists of England, Scotland and Wales.

The UK consists of Great Britain and Northern Ireland.

Answer 2

London is the capital city of England and the largest city in Great Britain. Britain doesn't technically have a capital city as the term Britain refers to the union of England, Scotland, and Wales as Great Britain, however London is to all intents and purposes the capital of Britain.