peer of the realm vs Britain

peer of the realm and Britain both are nouns.

Word	Noun	Adjective	Verb	Adverb
peer of the realm	Yes	No	No	No
Britain	Yes	No	No	No

peer of the realm (noun)	Britain (noun)
a peer who is entitled to sit in the House of Lords	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