U.K. vs post-free

U.K. is a noun but post-free is not a noun.

Word	Noun	Adjective	Verb	Adverb
U.K.	Yes	No	No	No
post-free	No	Yes	No	Yes

U.K. (noun)	post-free (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)	post-free (adjective)
	postpaid

U.K. (adverb)	post-free (adverb)
	having the postage paid by the sender