
This map looks good. I have studied the overlap of medieval kingdoms, dialect regions, DNA clustering, and the lexical footprint of surnames. This is withing what one would expect, with the use of zones in which groups bleed into the other.

The basis of the dialects in England date from around 600 or so, and correspond strongly with kingdoms.
You can see Ancestry's map has a zone overlapping with the Anglo-Saxon kingdoms, Mercia, and three coming out of the old Celtic kingdom of Dumnonia (Devon & Cornwall). I'm not sure what is delineating their Northern areas.
This genetic clustring shows Scotland in a bit more resolution:

A dialect map:
