For the aerodrome and approach courses how good do you have to be to pass! I appreciate that whatever you do has to be safe and reasonably expeditious but since these courses seem relatively short, I’m wondering how busy, in terms of aircraft movements, are the final simulator runs and how complex is the airport and airspace used in the simulators?
No disrespect but thats an odd question. You have to be good enough to satisfy the CAA's approved course.
I may be wrong in assuming your history, but if you have been to the NATS college you will find the traffic levels very similar.
They will provide the necessary training to enable you to deal with the traffic levels. Its entirely up to you and your ability whether you can manage them.
Sorry if I sound a bit harsh.
As VCR said, I would highly recommend you visit both colleges and have a look for yourself. Meet the instructors (they have probably worked in both colleges at some point) and get a feel for the place. If you are worried about traffic levels in a sim maybe you should take the aptitude test to ensure you have the required ability to begin the course.
Good luck