In reply to CL1MB0N:
In general, the structure of Welsh rock tends to "meander". Routes that go straight up splitter cracks, or straight up the middle of blank slabs, are few & far between. There's a reason why half ropes are standard for British trad climbing. Having said that, I'm sure somebody competent and experienced with extending gear for single rope trad climbing would find things to do.
St Davids: not that familiar with the area, but from what I've seen generally shorter routes and less tidal/abseil approaches than South Pembroke. Probably ok.
Tremadog: quite a few wandering pitches but probably feasible.
CLoggy, Dinas Mot: definitely not places I would really want to go to with a single rope, but probably doable if you choose routes with care.
Cromlech: ok
Slate: ok except provided you don't embarrassingly strand yourself at the top of the Dervish (40 metre abseil descent)