I agree entirely, but to remain different from other regions each would need to have little contact with other accents which would otherwise influence a change, slowly mixing the different accents, as is now happening in the UK. There are lots of very interesting examples around the world. For example, Norse was spoken throughout present day Norway, Sweden and Denmark and crossed the sea to Britain, Ireland, Faeroes and Iceland, plus other territories at some time conquered by vikings. Following a reduction in movement of people locally between Norway and Sweden they developed apart linguistically (remaining unmistakeably similar however), and Denmark even more so. The Faeroes stayed much closer to old Norse, but the place which stayed most isolated through the centuries, Iceland, still has a language that is virtually unchanged Norse. On the other hand, there is not really much difference throughout Iceland in terms of accent, even though distances are large and communities were quite isolated within Iceland. In the UK regional accents possibly have their roots in separate languages (including long dead indiginous ones) as you suggest.