U.K. vs top-hole

U.K. is a noun but top-hole is not a noun.

Word	Noun	Adjective	Verb	Adverb
U.K.	Yes	No	No	No
top-hole	No	Yes	No	No

U.K. (noun)	top-hole (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)	top-hole (adjective)
	excellent; best possible