UK vs all-mains

UK is a noun but all-mains is not a noun.

Word	Noun	Adjective	Verb	Adverb
UK	Yes	No	No	No
all-mains	No	Yes	No	No

UK (noun)	all-mains (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)	all-mains (adjective)
	used of a radio receiver that is adaptable to all voltages