U.K. vs Secret Intelligence Service

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

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

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