pillar box vs United Kingdom of Great Britain and Northern Ireland

pillar box and United Kingdom of Great Britain and Northern Ireland both are nouns.

Word	Noun	Adjective	Verb	Adverb
pillar box	Yes	No	No	No
United Kingdom of Great Britain and Northern Ireland	Yes	No	No	No

pillar box (noun)	United Kingdom of Great Britain and Northern Ireland (noun)
a red pillar-shaped letter box	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