Some Ancient Britons were in fact part of the island's indigenous 'Iron age' tribes, and were of a unique ethnic stock; they just assigned themselves to the Celtic language and, to a certain extent, were influenced by Celtic culture.
However, you cannot simply refer to all the Iron Age inhabitants of Britain as being of the of the same ethnic origin. As with any period of the island's history, Britain at this point was a seat of migration. Yes, Iron Age Britain did have aboriginal inhabitant, but many of her tribes