U.K. vs made-up

U.K. is a noun but made-up is not a noun.

Word	Noun	Adjective	Verb	Adverb
U.K.	Yes	No	No	No
made-up	No	Yes	No	No

U.K. (noun)	made-up (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

U.K. (adjective)	made-up (adjective)
	having been surfaced (e.g. with asphalt)