Secret Intelligence Service vs U.K.

Secret Intelligence Service and U.K. both are nouns.

Word	Noun	Adjective	Verb	Adverb
Secret Intelligence Service	Yes	No	No	No
U.K.	Yes	No	No	No

Secret Intelligence Service (noun)	U.K. (noun)
the government agency in the United Kingdom that is responsible for internal security and counterintelligence overseas	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