The Angles, Saxons, Jutes, and Frisians considered themselves separate ethnic groups although they were aware that they were closely related to each other and that their dialects were mutually intelligible. The Swedish and Danish also considered themselves separate ethnic groups despite acknowledging how close they were.
Similar to the above, was it the same for the ancient British? Did the ancient Cumbrians, Welsh (Cymraeg), Cornish, etc. all see themselves as separate ethnic groups (like the Frisians & Saxons did) but acknowledge that they were closely related Brythonic peoples? Or did the Cumbrians, Welsh, etc. see themselves as part of a single ethnic group with the only difference being geography? I was wondering since the terms Cymraeg and Cumbri sound very similar and come from the same root and I would think their languages would also be mutually intelligible in ancient times.
This post was made earlier in the day, and still hasn't received any responses. I suspect this is due to the difficulties in answering such a question. To be able to answer it we would need enough literature from them to glean from it how they saw each other. Given that there is a distinct lack of written material, any answer would by necessity be speculation. It's probably not what you want to hear, but the short answer is that we really have little way of knowing.