U.K. vs British Isles

U.K. and British Isles both are nouns.

Word	Noun	Adjective	Verb	Adverb
U.K.	Yes	No	No	No
British Isles	Yes	No	No	No

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