Agreed. But you could make similar arguments about other unrepresented peoples in most other parts of Europe. The Celts are just better known and more highly romanticized. If you wish to contest this, I invite you to America to see thousands of drunken fraternity members, ignorant of their dubious heritage, wearing kilts.
Seriously, the Celts did get consideration, as did the Normans - the Normans, I think, were the last possible civ we eliminated (as they had important roles in France and S. Italy as well.) There's just not enough room. The UK regions are tricky to balance - the channel makes invasions infrequent and difficult, and the region really
should have a lot of resources - but if you give them too many, and no competition, England will always be a dominant power in the game. Conversely, as there isn't much space to expand, the English game is limited in terms of conquest options (with big stability penalties for controlling areas of continental Europe.) The best balance seems to be giving an independent, proto-Celtic civ Edinburgh, York, Ath Cliath, and possibly a small Welsh city, and forcing England to expend its energy early securing its core region.