Gotta agree with the other chaps/chapesses on here. First make sure your medical is in place, then look at which one to go for.
For Tower/Basic, I'd recommend Cwymbran as they have the 360 degree Simulator which is excellent.
I've only been to ASTAC for my courses(Tower, Approach and Approach Radar. They were all excellent, instructors great and college atmosphere with staff and fellow students was superb. Hear the NATS college is good too, but the courses are bigger and hence will have a higher student instructor ratio and as a consequence does suffer a higher fail rate.
Traffic ... well, it's busy enough at every stage to make you work ... hard! Each college has to satisfy the same requirements from the CAA so are identical in that respect. Go on a visit to each and see which one you prefer. Don't worry about the traffic, you build up to each level. But once you pass it shows you have the ability to go on to "real world" traffic for further training.
There is no easy back door into this one!!