Hello Ralf, I'm in the Northern Section, so can't help directly. (Can't Martyn suggest another course member as a buddy?)
If you wanted to dive something in Wales or on Mendip (i.e. Somerset area) it might be best to contact those Sections directly (details on the CDG website) - can't promise but it may be that someone would offer to sort something out. I guess it depends on someone having the time available on the right days. Whether or not you already have caving experience will influence what is / is not safely possible. It's definitely best to dive UK sumps via contact with the local CDG members; the conditions aren't really much like what a "cavern" course deals with. Martyn is a dyed-in-the-wool CDG member so he can probably point you at the right people if you ask in advance (so there's more time to make arrangements).
Maybe someone in those areas will see this and send you a PM
Best of luck anyway.