"British" = English, Scottish and Welsh. The Co-op is yet another supermarket which sees fit to recognise Scotland and Wales but totally ignore England.
UPDATE: "Cornubian" has written to say that Cornwall is also excluded from the national flags on the packaging at the Co-op. Interesting point, although personally, until there has been a referendum on the subject and the people have actually voted for independence for Cornwall, I do not believe that Cornish produce should carry the "Cornish" flag - unless it is coupled with an English one. Being half-Scot myself, I have grave concerns about the whole exclusive "Celtic" thing which is so much a part of Cornish nationalism.