My understanding was that there was very little actual evidence of what was spoken in Britannia prior to the Roman invasion. The Romans basically noticed they were Celts (well, Galli) and told us the names of the tribes and not much else and inscriptions etc are almost non-existant. (Ogham being a mainly Goidelic thing with the exception of some weird inscriptions in Scotland in what might be Pictish.) Toponyms might give a general idea of what was being spoken I suppose but I'm fairly clueless in that area. At any rate there were Belgae in the South East when the Romans turned up and I read a while back that the jury was barely in on whether they spoke a celtic or germanic language. I'm not saying that Brythonic Celtic isn't the most probable or anything, just that the actual evidence is patchy (as far as I know-once again I'm no expert.)