I was about to say that Peter Sallis (the voice of Wallace, and also a member of the cast of the series Last of the Summer Wine, set in Yorks) may have been from Lancs, but I just discovered he is in fact from Twickenham, so bang goes that theory.

However, Aardman Animations is based in Bristol, so if he went there to do the voice-overs he could have unconsciously picked up the local accent :-)