The standards that each college has to meet are identical, and they are assessed and regulated by the same branch of the CAA, however I wouldn't be surprised if the pass rates for both Rudloe and Cwmbran are better than those at CATC for the courses that they have in common (ie. ADV/I and APS).
This may be to do with the smaller course sizes and having more contact with a smaller number of instructors throughout the training. From personal experience of both systems, on a course at one of the private colleges I had one or two different instructors who knew me well and knew exactly what my weak points were and what I needed to work on. On a course at CATC, I had upwards of 10 instructors (sometimes significantly more), some of whom I sat with reasonably often, others of whom knew nothing about me and had only the comments on the report from the last sim run to go on.
I just have never heard much about ASTAC. I'm assuming that, unlike NATS, you have to pay to attend the college, rather than them employing you??
That is correct (although you can also pay NATS to train you, if you really feel like it). The private colleges exist mainly to train controllers for the ATC units that are not run by NATS, and there are many of them.