Explain how they are an ethnic group when the name "Briton" or "Brython" itself has never referred  to all the peoples of the British Isles. Most people in Scotland called themselves Picts, Gaels/Scots or Anglo-Saxons in the past depending on where they were from. Irish have never called themselves Brythons or Britons and still do not. Briton is a political term relating to the country fo Great Britain that has only existed since the 18th century. Britons are not some distinct collective ethnic or cultural entity. This whole article does not relate to any British ethnic group and anything relating to a ethnic group appears to be minority opinions and original research (see WP:No original research).