I'm aware of the possible dubiousness of using the term 'Celts' before the 18th C. However, it is a modern label that has stuck as a description of the indiginous Britons, and British Celts is less of a mouthful than 'Iron Age Romano British'. It is feasible that the Britons were a Celtic influenced culture, despite modern, personal, theorising. It's accepted that there was no such thing as a Celtic 'race', (other than the links through the pan-European Haplogroup Rb1), with the Celts being more of a common culture than a common blood. The transmigration of Celtic language and religion into Britain is evidence that this culture spread and survived here. Therefore it's not at all wrong to use the term 'British Celts', though there is a degree of deceptiveness involved in using that term. I will continue to use the term.

No 'hard evidence of invasion' does not mean no Celts in Britain. English speaking countries of today speak English because the English actually went and lived there through war or banishment in the past. That obviously doesn't make English speaking Navarro Indians of English descent, but it suggests cultural contact. Is Dr Simon James saying there were never ANY Celts in Britain? Where is the hard evidence for that? Balancing his personal theory with the many others that exist, the balance of probability is that there was a strong Celtic influence in Ancient Britain, even if it wasn't overwhelmed with a race of people called Celts.