Um, I'm not sure that there's any proof that all the celtic languages in Britain were from the Brythonic (or p) branch of celtic. (Although it seems more likely than goidelic; still no particular reason to believe they would have been particularly close to Welsh other than proximity.) I've a vague memory of reading something that said the Silures were rather different and insular and I think they would have been the main tribe in the Welsh Cornish neck of the woods. Obviously scottish gaelic was/is goidelic, but apparently it came over from Ireland with the Dalriada. If someone knew anything about cumbrian, which I have a feeling was fairly well documented I'd be interested. Gaulish was apparently surprisingly close to latin according to the romans, so there's a distinct possibility that a lot of 'celtic' particularities only sprang up in Britain and Ireland.

Not an expert in any of this stuff- feel free to correct me.