Lots of great driving roads in south wales in the Brecons!
Firstly, the B4560 from Beaufort to Llangynidr - road over the moors which is frequeny used in Chris Harris videos - theres a hairpin that features in nearly every one of his videos. Also in the same area the run up the hill from Llangattock is brilliant.
Secondly, the A4059 running north from Hirwaun to the A470 - brilliant moorland road, can be very fast with well sighted sweepers. Doesnt really go anywhere either, so is good and quiet. Only thing is watch for sheep! so good needs to be ran in both directions!
Finally from me, the A4069 from Llangadog to Brynamman - although this needs to be early in the morning as it will be heaving with caravans, and its a bit too narrow to overtake safely.