I can't say how common the usage would have been in 1921, but yes, there is a 1921 newspaper reference mentioning a proposal to substitute North Britain for Scotland and South Britain for England. There is also the likes of this 1929 newspaper reference:
NOTICE IS HEREBY GIVEN that all Persons having any CLAIMS or DEMANDS against the estate of [NAME], deceased... formerly of Geldie Cottage, Braemar, in the County of Aberdeen, in North Britain ...