British spelling vs theorise

British spelling is a noun but theorise is not a noun.

Word	Noun	Adjective	Verb	Adverb
British spelling	Yes	No	No	No
theorise	No	No	Yes	No

British spelling (noun)	theorise (noun)
English orthography used in the UK, Ireland and most of the Commonwealth (characterized by 'analyse', 'centre' and 'useable')

British spelling (verb)	theorise (verb)
	to believe especially on uncertain or tentative grounds