island vs British Isles

island and British Isles both are nouns.

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

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