DrWatson
Wow, masses of good ideas! Not so many for Somerset though, and there are lots of great locations, Wells is one, though it has no trains (if that might be important). I'd suggest somewhere just outside Taunton, perhaps?
In Dorset, goodness, so many great places, you'd need to decide if the tourist crowds for chunks of the year will be a nuisance, and if not, which end of the coastline (assuming seaside is good), you'd prefer? Over by Poole & Bournemouth, or near Dorchester & Weymouth, along by Bridport & West Bay, or even further west by Lyme Regis or Charmouth? Swanage or Wareham perhaps? Or inland, Sturminster, or Sherborne, that had a vote or two, and it's very nice. If you like shows, lots of good acts appear at the Bournemouth International Centre.
I'd actually suggest somewhere near Exeter (yes, Devon I know!), a terrific city, but lots of smaller places within easy reach, Exmouth, Topsham, Teignmouth, Newton Abbot and more. Jolly good luck!
I’d actually vote against Charmouth, steep hill, limited parking and residents not welcoming to all. Our neighbour regrets moving there, though it looks OK on paper.