The British Celts were themselves relatively recent arrivals in the British Isles. They would still have had similar languages and customs with their distant relations in Gaul. There was a thriving cross-Channel trade at the time of Caesar's conquest run in large part by the Veneti. They certainly would have been able to translate for Caesar if the more southern Gauls like the Aedui were not able to understand the British Celtic dialect. Also, and I may have to double-check my Caesar's Commentaries when I get home, wasn't one of Caesar's justifications for invasion that the British Celts were offering support and sanctuary to the Gaulish Celts fighting against him? If so, that suggestions an even greater likelihood that the British and Gaulish dialects were mutually intelligible.