Suggestions for Home Base in Southwest England (Somerset, Devon, Dorset, Bristol) ?

Glastonbury is difficult to reach & not worth it in my view.

I would suggest you go direct from London to Cardiff by train and stay maybe 2 nights. See St.Fagans Castle & Museum, Cardiff Castle & Caerphilly Castle just to the north.

Consider Bath as a second base as it has direct (slow) trains down to the Dorset coast at Weymouth. (These also go from Bristol). If having a private tour of Jurassic coast, it is best if you leave the train at Dorchester West - as your guide should explain.

Rather than Bristol, have you thought about Bath instead? It is an attractive Geiorgian city with an Abbey and best of all, the Roman Baths.

You can do Cardiff by train froiom Bath and it is a regular service (depending on the train you choose, you may need to change at Bristol) Buy tickets in advance for the best prices.

https://www.gwr.com/tickets/#clear

There is a regular bus servive to Wells. Check timetables here.

https://bustimes.org/services/173-wells-midsomer-norton-bath-city-centre

https://bustimes.org/services/174-wells-midsomer-norton-bath-city-centre

If Glastonbury is a must (and many people don't think so) then catch a bus from Wells to Glastonbury

https://bustimes.org/services/376-bristol-pensford-wells-glastonbury-street

James and Wasleys - Thank you for your input. I am very keen on old and interesting architecture and hence my interest in Glastonbury Abbey and Wells for the Cathedral.

Medieval architecture: if that’s the case, I would suggest that you visit Tintern Abbey (ruin) which is in the Wye Valley north of Chepstow. https://cadw.gov.wales/visit/places-to-visit/tintern-abbey
To get to this place, direct trains run roughly hourly from Cardiff or if coming from Bristol/Bath - change at Severn Tunnel Junction. (Pay on the day for these short hops). www.traintickets.com will tell you times & fares.
You will need a bus from Chepstow to Tintern. Traveline.info will tell you.
If using a car from Bristol/Bath, use the M48 Severn Bridge.

Chepstow also has a medieval castle about a 12 minute walk from the station. (Also see from the river bridge).
https://cadw.gov.wales/visit/places-to-visit/chepstow-castle

If staying in Cardiff, you might also like to see:>https://www.llandaffcathedral.org.uk

AS well as Wells Cathedral, don't forget the Bishop's Palace next to it complete with moat and swans that have been trained to ring a bell to be fed. On the opposite side of the Cathedral is Vicar's Close, believed to be the only complete medieval street left in England and a rare example of a planned Medieval street.

We stayed in Bath and did a day trip to Glastonbury and Wells one day.

On leaving Bath, we headed to S. Wales, visiting Tintern Abby, then a couple of castles on our way to Cardiff. In Cardiff, we visited Cardiff Castle, took a short train ride to see Caerphilly Castle and upon leaving Cardiff stopped and visited the Museum of Welsh Life just west of Cardiff.