Dr Merrony confirmed his suspicions of a Roman fort by looking for Roman tilingMark Merrony
Thar she blows!
That said, at some point after the Roman legions left and after invitation of mercenary Saxons like Horst and Hengst, what was left of Roman and Christian Britain was crowed into Cornwall and Wales. At that point the original non-germanic Britons would probably be more inclined to consider themselves "Britons". See . Gildas the Wise, "De Excidio et Conquestu Britanniae". (Pretty sure he would have a few things to say about their current barbarian invasion and having become a "Lapsed" non-Christian country.)