British English vs fill in

British English is a noun but fill in is not a noun.

Word	Noun	Adjective	Verb	Adverb
British English	Yes	No	No	No
fill in	No	No	Yes	No

British English (noun)	fill in (noun)
English as spoken in the UK

British English (verb)	fill in (verb)
	supply with information on a specific topic
	write all the required information onto a form
	represent the effect of shade or shadow on
	be a substitute