Hey rac3r,
I'm currently going through NATS selection at the minute. With regards to preperation, I don't think there is really a huge amount you can do that will be terribly helpful for selection at your stage. Obviously some background study in air traffic control will help fuel your enthusiasm, and may stand you in good stead if you get through to the training, but very little of this will have any relevance to the selection process (might look good on your application form though
). Most of the selection exams are things like spatial awareness and numeracy rather than actual questions about air traffic control (there is one exam on this, but it is based entirely upon a document provided by NATS rather than any books or other info, check out
http://natscareers.co.uk/docs/test_materials.doc).
If you're not too good at maths then it might help to start practicing your mental arithmatic and speed/distance/time calculations. This is fairly basic math so even if you struggled at school you can probably overcome this obstacle if you put a bit of work into it. You need to be able to do fairly simple math quickly and accurately rather than having to do anything complicated. A good website for practice questions is
Speed Distance Time | OASC: RAF Officer and Aircrew Selection Centre there are loads of examples here that are very similar to the kind of questions I've been asked so far in selection, and this website also tells you how quickly you are working (try and aim for an average of less than 30 secs per question once you get onto the tougher ones).
For more info on the selection process, and the official advice from NATS for preparing for each stage, check out
Application process - Jobs and careers at NATS
Apart from that there's not really a lot you can do until you've finished your degree and are ready to start selection. I'd definitely encourage you to focus on your course for the time being rather than getting too worried about the ATCO stuff. I don't think NATS are particularly interested in qualifications (past GCSE) and your degree will give you no specific advantage in that regard, but if you are used to working hard and focusing on your studies then this will be a useful and transferrable skill that will serve you well. If you are really keen then it might be worth starting your application before you are finished your course as the selection process can take several months... but once you're started it can be quite hard work so if it were me I'd wait rather than risking screwing up my finals because I'm concentrating on ATCO selection.