Good question.
The term British as applied to current residents of the Pretanic Isles, is also a latter day term, implying that the "united" kingdoms of England, Scotland, Ireland and Wales were to be regarded as the inheritors of the term Briton, being collectively descended from them. Which is not so.
The Brythonic's descendants now live largely in North Wales. Some went to Eire at the sacking of Anglesey c.65 AD.
The English are not Britons, Brit's. Some of them may be Brit-ish (as in cold-ish, just a bit).
A bit of Brit in the mix.
Answer to your question is;
Citizen of the U.K.