British capital vs Greater London

British capital and Greater London both are nouns.

Word	Noun	Adjective	Verb	Adverb
British capital	Yes	No	No	No
Greater London	Yes	No	No	No

As nouns, British capital and Greater London are synonyms defined as:

British capital and Greater London: the capital and largest city of England; located on the Thames in southeastern England; financial and industrial and cultural center

Other synonyms of British capital include capital of the United Kingdom, London.

British capital (noun)	Greater London (noun)
the capital and largest city of England; located on the Thames in southeastern England; financial and industrial and cultural center	the capital and largest city of England; located on the Thames in southeastern England; financial and industrial and cultural center