You have Bike Park Wales at Merthyr Tydfill. There is loads in South Wales. Alternativly in the Forest of Dean there are is a nice centre with a load of trails running by it. I've never hired a bike anywhere though so I cant recommend anyone specifically for that. If you get the bug and buy a bike, ffs buy something sensible and save yourself a fortune in the long run. Dont even think about buying an apollo or an equivalent.
http://www.pedalprogression.com/ Ashton Court Bristol
Good trails, no idea on the rental service though as I havn't used it.