British Isles vs island

British Isles and island both are nouns.

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

British Isles (noun)	island (noun)
Great Britain and Ireland and adjacent islands in the north Atlantic	a zone or area resembling an island
	a land mass (smaller than a continent) that is surrounded by water