I know a colony was, I suppose, accidentally set up in what is now Brittany, when the Legions left Britain with Macsen Wledig, at least, I believe that is what happened. So, whatever the local Gauls spoke, the incomers from Prydain, whether legionaries or Brits who wanted to stay in the Empire, will have spoken a Latin/Prydainig mixture. I understand there is a lot of Latin in Breton. A very good friend of mine, about 5 years my senior, said he remembered onion sellers in Whitland being understood, presumably just after WW2, but whether they spoke Breton or leaned some Cymraeg, who knows? Cornwall spoke British when we all did and was later getting conquered by anyone else, but, given the differences between North and South Wales, I'll bet Cornish was pretty unique. This week on Great British Menu, a chap from Cornwall did say soup,is cowl! (Which he pronounced as we say 'cawl')!