the three estates vs United Kingdom of Great Britain and Northern Ireland

the three estates and United Kingdom of Great Britain and Northern Ireland both are nouns.

Word	Noun	Adjective	Verb	Adverb
the three estates	Yes	No	No	No
United Kingdom of Great Britain and Northern Ireland	Yes	No	No	No

the three estates (noun)	United Kingdom of Great Britain and Northern Ireland (noun)
a major social class or order of persons regarded collectively as part of the body politic of the country (especially in the United Kingdom) and formerly possessing distinct political rights	a monarchy in northwestern Europe occupying most of the British Isles; divided into England and Scotland and Wales and Northern Ireland; `Great Britain' is often used loosely to refer to the United Kingdom