British Isles vs Britain

British Isles and Britain both are nouns.

Word	Noun	Adjective	Verb	Adverb
British Isles	Yes	No	No	No
Britain	Yes	No	No	No

British Isles (noun)	Britain (noun)
Great Britain and Ireland and adjacent islands in the north Atlantic	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