As Night Crawler says above the Nant Y Gro trials were explosive charges set in position and detonated.
The first airborne drop trials were at the River Fleet at Chesil Beach, only of reduced-scale models, but by the time a fullsize "Upkeep" bomb was tested they had moved to Reculver in Kent. The Barnes Wallis exhibition at the Brooklands Museum includes the remains of one of the Chesil Beach test weapons alongside a real Upkeep. I think the Derwent Dam reservoir was only used for crew training with the triangulating bomb sight on the towers.
The Brean Down site was used to test a rocket-propelled version to be launched from a ship. The launch ramp is still there, beyond the Palmerston Fort (National Trust) and WWII gun and searchlight emplacements. Local legend has it that the site was only used once as the stops, intended to retain the launch trolley, failed to stop it and the lot vanished into the Bristol Channel.
Nat Y Gro is worth a visit - only 3/4 mile walk from the Elan Valley visitors car park and the dam is still there, though the edges of the breach have been tidied up with grinders so there are no sharp bits of rebar sticking out. In the river bed you can still see the concrete spillway which can be seen being washed away from the dam in the film of the test
