u vs UK

u and UK both are nouns.

Word	Noun	Adjective	Verb	Adverb
u	Yes	Yes	No	No
UK	Yes	No	No	No

u (noun)	UK (noun)
the 21st letter of the Roman alphabet	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 (adjective)	UK (adjective)
(chiefly British) of or appropriate to the upper classes especially in language use