pillar box vs United Kingdom

pillar box and United Kingdom both are nouns.

Word	Noun	Adjective	Verb	Adverb
pillar box	Yes	No	No	No
United Kingdom	Yes	No	No	No

pillar box (noun)	United Kingdom (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