Great Britain has not had a flag since 1800. The flag comprised St George's cross (a centred red cross on a white background representing England and Wales, which had been unified in the sixteenth century) and the Saltire (the flag of Scotland; a diagonal white cross on a blue background).
The Union Flag (the flag of the United Kingdom) which came into being on 1 January 1801, added to the above St Patrick's cross, a diagonal red cross on a white background. Until 1922 St Patrick's cross represented Ireland. It now represents Northern Ireland as the fourth part of the United Kingdom.