national capital vs capital of the United Kingdom

national capital and capital of the United Kingdom both are nouns.

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

national capital (noun)	capital of the United Kingdom (noun)
the capital city of a nation	the capital and largest city of England; located on the Thames in southeastern England; financial and industrial and cultural center