Do you think the clause "only when certain of a person’s heritage" is complete?
Residents of any of these countries may be called "British." Use "English," "Scot" or "Scotsman," "Welsh" and "Irish" or "Northern Irish" only when certain of a person’s heritage.
I suppose that "is concerned" could be omitted in the clause "only when certain of a person’s heritage". How do you think about it? Thank you!