The NATS selection process is pretty tough... but you've got to bear in mind that ATC is no normal career. NATS are looking for people with a very specific set of attributes that (in their experience) seem to be the most important for the job. I don't mean to be elitest or the like, but if you are worried you might not have what it takes for NATS then are you sure that this is the right career to be thinking about? I honestly don't mean to put you off (there are enough people on these forums who will quite happily do that without me adding to their number), but just want to encourage you to think this through properly before you start committing yourself to anything.
Obviously I can see what you are saying about NATS selection being quite limiting, but in other ways it is very open. I can't think of any other job where you will almost certainly end up earning in excess of 60-90k that is so easily accessible to people without any formal qualifications. Technically you can pass NATS selection with nothing but a couple of GCSE's. I'd say that was quite broad and not very limiting at all.
I'd also point out that the NATS selection isn't actually competitive. At each stage in the selection process there is a benchmark you are expected to meet, if you do so you will be invited along to the next stage, it has nothing to do with competition or the performance of any other candidate.
There are a few other routes into ATC, like self funding your own training at an indepedent college. As far as I know there are only two independent colleges training air traffic controllers in the UK, which are ASTAC (which you appear to know about) and Cwmbran (in Wales, run by BAE). Self funding is risky though, you are by no means gauranteed a job at the end of your training, and you'd probably be limited to aerodrome rather than area controlling (NATS are currently the only route into area control in the UK, and are likely to remain so for a very long time to come). Another option would be to look into becoming an ATC assistant and hoping that your employer might one day consider sponsoring you for full ATC training, but of course there are also no gaurantees here - assistant jobs are quite hard to come by and this is only likely to get worse as advances in technology reduce the requirement for the role, and with quite a number of fully trained ATCOs looking for work most aerodromes will likely look to fulfil their future ATCO requirements from this pool than to invest in training new ATCOs up at their own expense.
If you don't mind going abroad you could look into Eurocontrol, but their selection is very much like NATS.
Basically, NATS are the big fish in the UK, they run all the area control and most of the major aerodromes are contracted to them as well. The vast majority of UK ATCOs are currently employed by NATS, so if you are really interested in this job they are the main way to go. If you are worried about their selection process then just give it a go and see what happens, you might suprise yourself and you certainly aren't going to loose anything by having a crack at it.
Best of luck
