Russ,
Celeste and I did Wales this spring after seeing our daughter in St. Andrews, Scotland, and we drove just about everywhere. Rented a car in York, and returned it in Chester. Don't attempt to drive or rent in London. The English M roads are our freeways, and most A roads in England and Wales are not difficult. Some of the A roads in Wales and Scotland, and ALL B roads are frightfully narrow. We actually stayed across the bay in Harlech (just outside of the Castle, which is also a MUST SEE) and drove into Porthmadog each day to ride the train. The drives in and out of Porthmadog are also spectacular. We also enjoyed Portmerion village where the TV show The Prisoner was filmed and for the non rail oriented it is worth the visit.
The only thing that I would tell you is to budget at least one more day than you plan for. We could have easily stayed another day and simply ran out of time, but it gives us a reason to go back.
I want to do the Brecon Mountain and the Vale of Rheidol railways as well as the others already mentioned then as well.