Abz217,
I've only just done Stage 1 so I don't know facts about the college. However, I would hazard a guess that something like half the people don't make it through that it's likely to be slightly taxing.
I would definitely say finish your A-Levels. At worst it will be good exam practice, but assuming you don't make it as a controller you'll probably have to go back and do them again ifyou then decide that you are going to university. I studied engineering at uni and had to do a foundation year thanks to not doing very well in my A-Levels, so I've been there. I spent all of my time working during A-Levels. I was earning more back then than some of my friends from uni were after graduating so you don't necessarily need a degree to get reasonably well paid, but for the life experiences I would recommend it. Just remember it will cost you years of your life to get back to where you are now if you decide to drop out.