I did a search for South Wales in this section of the forum before going on vacation there and didn't find anything, so this is my first contribution to the Locations section.
South Wales has a lot of opportunity, but is also very challenging. The main issues will be weather and composition. Many shots have human pollution in them (cities, buildings in background, etc). There are also a lot of beautiful fields here but you can't see any of them from the roads because the hedges are taller than your car.
Beaches
There are quite a few beaches and cliffs that make good subjects around the Gower Peninsula. In Mumbles, Bracelet bay has some interesting rock formations good for leading the eye and helping frame the shot, depending on the angle you shoot at. Look for a restaurant called Castellamare at Bracelet Bay along the Mumbles Road. On the patio you have a good view of a lighthouse there with Swansea in the far distance. The best shot I've seen of this so far is from a hilltop on the path leading out to the lighthouse. The path starts in a parking lot.
Also check out Langland Beach, Three Cliffs, and Worm's Head. Worms Head is interesting but very challenging to turn into a good composition. There's a very long beach next to it which could also make for a nice shot, but when I was there the haze was too strong. Early morning would probably be better.
Waterfalls
Skip Aberdulais falls. The tin processing plant is interesting history, but the falls themselves have too much human pollution to get a good comp of. There is, however, an interesting falls a bit down the road and a short walk from a car park. It's very tall and you'll be shooting relatively close, so it would be a good scenario for a tilt-shift lens to correct the perspective.
Sgwd Y Pannwr in The Brecons is definitely worth hiking to, and there are a lot of other waterfalls there as well. Think Yosemite but at a much smaller scale, although it's still quite a bit of a hike and will take a few days to go around and see them all. There are a lot of waterfalls around Neath, so google that.
The Brecons
I'm in California and to me these are like very large hills rather than mountains. Still, the view as you come into them from the south is absolutely stunning on a good day. Perfect motorcycle country for those that ride, it reminded me of open hilly ranges of Montana. The sheep in the middle of the road were a nice touch. The best shots from the Brecons seem to involve climbing the hills early in the morning for sunrise. Climbing Pen Y Fan, the tallest one, takes about 2-3 hours on the walking path and is 1700ft of elevation. There are a lot of freestanding stone walls in the Brecons which can make good shots leading the eye to the mountains and valleys in the distance.
Speaking of the sheep
I feel like the sheep and cows on the Gower could make good subjects in the right situation. You should know ahead of time that the sheep all have paint on their backs to mark ownership. The cows are not penned in and will wander around a large area and even go into front yards and eat all the flowers. You can find them just about anywhere away from the main town centers.
The roads
Look for the windy rural roads, especially ones that go across country through undeveloped areas (check google satellite). Some of them would make wonderful road-to-the-distance shots. There's one that goes down from the top of a hill and then rises and falls over a series of hills like a roller-coaster. I thought it was a bit similar to the typical shot of the highway descending down into monument valley, but much greener and with sheep.
The castles
Forget these, for the most part. There's one near Pennard Gold Club which you can see from there, and it overlooks one of the sandy beaches and a gorgeous winding stream, but the angle of the view, etc, makes composition a challenge. Expect all ruins to have huge signs in the shot. Oystermouth in Mumbles is very interesting to visit as a tourist, but forget about any fine art photography here.
Gear
Two things I'd recommend for the UK in general: a tilt-shift lens, probably 24mm, and a 4 or 10 stop ND filter for long exposures. I suggest the tilt-shift because everything seems close-up if it's tall so you need the shift or because the landscape shots need close-to-far focus that you get with tilt. The ND filter (and a could of grad NDs for the sky) would be good for blurring those poorly-defined grey clouds into a perfect grey smear.
Weather
Expect grey clouds a lot, usually at multiple levels in the atmosphere, meaning they'll have different levels of brightness. Also expect quite a bit of wind. If you're trying for crispness and detail in your shots, you'll have a challenge waiting for the wind to die down long enough to get the exposure.
