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!

Question

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!

Answer

No, it would need 'is concerned', it is not a complete sentence otherwise.

Answer

Because "when" is a conjunction, it should be followed by a sentence.  “ Only when certain of a person’s heritag“ is a phrase,  so I suppose that it could be written like this "...only when certain of a person’s heritage is concerned".

Answer

I'm confused by your question. 
The phrase "is concerned" is not in the clause.   It also doesn't belong there.  The sentence is fine as is.