The Welsh living in Wales
Celtic isn't native to the british isles. in fact the celts spent a few hundred years spreading out from central Europe in big migrations, covering a huge chunk of western europe. Who were the pre-celtic people of the british isles? What was their language? What did they call themselves?
Xant's point is that any collective tribal identification that survives to this day only does so because of war. This includes language groups. "Abberations" like basque only survived because of geography. Like kurdish and other isolated minorities that are identified primarily with mountainous or hilly territory, or other isolated types of terrain (deserts, out of the way insular, etc) . They weren't always, of course, that's just where the remnants eventually ended up, because the territory was easier to defend or just not worth conquering at all, as the rest of their people were displaced or assimilated. It wasn't some total population replacement genocide all the time (although you'd be surprised at how many were, the genetic data bears it out in certain cases), but think of how many tribal identifications in Europe alone have dissapeared entirely over the last, say, two thousand years. Don't even get me started on the MidEast and the deliberate obliteration of the many pre-Islamic cultures/languages that had existed for thousands of years prior to the arab eruption from the peninsula.