UK vs top-flight

UK is a noun but top-flight is not a noun.

Word	Noun	Adjective	Verb	Adverb
UK	Yes	No	No	No
top-flight	No	Yes	No	No

UK (noun)	top-flight (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

UK (adjective)	top-flight (adjective)
	excellent; best possible