first estate vs Britain

first estate and Britain both are nouns.

Word	Noun	Adjective	Verb	Adverb
first estate	Yes	No	No	No
Britain	Yes	No	No	No

first estate (noun)	Britain (noun)
the clergy in France and the heads of the church in Britain	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