I am a corporal in the ATC and so can give you some fairly decent knowledge about what to do when you're there.
I'm not sure how much you know about the opportunities available but here are some of the things i would defiantly recommend if you, like me are looking for a career in aviation military or commercial.
At 16 you can undertake a gliding scholarship which gives you approximately 8 hours of training before you do a solo circuit if you are considered good enough. Then at 17 you can do a flying scholarship which is similar however in a powered aircraft. Both of these, beside the Air Experience Flights give you a good idea about whether you do fancy a career in aviation and if so how well you can take on what the instructors tell you. This is also a great opportunity to speak to pilots about your future career hopes.
You should also work towards becoming a Noncommissioned officer, starting at a corporal and working upwards, this will look great on a CV.
As you mentioned, you can get a BTEC in Aviation Studies, this requires you to become a 'staff' cadet. To become this takes just over 2 years depending upon how much you can take in the information and how well you do in your exams.
All of this should set you up for a great future. Good luck in the Corps.
doogle92