usage vs Britishism

usage and Britishism both are nouns.

Word	Noun	Adjective	Verb	Adverb
usage	Yes	No	No	No
Britishism	Yes	No	No	No

As nouns, Britishism is a hyponym of usage; that is, Britishism is a word with a more specific, narrower meaning than usage:

Other hyponyms of usage include Americanism, Anglicism, consuetude, couvade, Germanism, habit, use, hijab, rite, ritual.

usage (noun)	Britishism (noun)
the act of using	a custom that is peculiar to England or its citizens
accepted or habitual practice	an expression that is used in Great Britain (especially as contrasted with American English)
the customary manner in which a language (or a form of a language) is spoken or written