@Ken Zayn – since United Kingdom and Great Britain are synonyms (AFAIK), it would be Briton.
Yep – just confirmed: the official name of the UK is:
“United Kingdom of Great Britain and Northern Ireland”
Thus, a person from that country would be either a Briton – or a Northern Irishman, I would guess, but I may be incorrect there, as Wikipedia lists this:
Demonym: British or Briton
Without leaving anything separate for the Northern Irish…