Eh. "Celts" is an extremely sketchy culture. AFAIK for Archeologists it mostly refers to neolithic through early iron age people from like, what, I think like 1000 bce to maybe shortly in to the current era? It's really down to some trends in pottery and a language group. Like technically "Celts" can refer to almost all of europe at different periods.
That said, the British sorting hierarchy of historical English perfidy goes Germans > Normans > Saxons > Angles > Britons > Dawn People > Whoever came before them. Probably giants or something.
Also the Dark Ages didn't real, it's just Victorian historians being arrogant shits like they usually are.