Cwmbran
Yep, I know them both well (ASTAC and Cwmbran) and there is not much to choose between them. A lot of military use the college to convert to a civil licence. I think the instruction at the colleges is more tailored to ATC assistants with some experience or ex-military controllers. Instructional levels vary but are OK. in my opinion - ASTAC seems ahead in the 'hit parade' at the moment (based on personalities previously mentioned in this thread) An assessment will give you a clue, but I know they will not necessarily give an accurate measure of your ability. Working in ATC as an assistant will give you a better clue. If it is non-NATS, they will sponsor you through one of the colleges after a while (most units are short of controllers). Apply to NATS before you do anything else is my advice. Good luck.