United Kingdom vs pillar box

United Kingdom and pillar box both are nouns.

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

United Kingdom (noun)	pillar box (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	a red pillar-shaped letter box