Greater London vs capital of the United Kingdom

Greater London and capital of the United Kingdom both are nouns.

Word	Noun	Adjective	Verb	Adverb
Greater London	Yes	No	No	No
capital of the United Kingdom	Yes	No	No	No

As nouns, Greater London and capital of the United Kingdom are synonyms defined as:

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

Other synonyms of Greater London include British capital, London.

Greater London (noun)	capital of the United Kingdom (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