Hi Ash
I found your question online and wanted to give you a quick reply. I am a senior F1 engineer and I get asked these sorts of questions all the time by friends or family members. I've set a up a blog where I'm writing some pieces on what to do in order to give yourself the best chance of getting a job in Formula 1 (or any motorsport for that matter), sharing my own experiences and my knowledge of recruitment from this side of the fence.
To answer your specific questions, I presume you are doing some kind of technical subjects for A-Level, namely Maths & Physics ? These subjects was correct as these pretty much fundamental to any engineer level job within F1 these days. The replies from Jersey Tom and copperkipper were pretty accurate I would say.
At this point there is no need to specialise at all beyond the above. Most designers and race engineer types in F1 have studied mechanical engineering, and even if they haven't then no-one will turn you away for choosing Mechanical Engineering.
If you are certain that you wanted to pursue aerodynamics as opposed to mechanical subjects then you could do an Aero course at University but that will pretty much send you down that path and then mechanical type design will be something you would need to cut across to which might not be that easy.
Personally, your degree is just a measure of your actual intelligence and aptitude for mathematical understanding. The subject is actually secondary. If you can get a good degree in a relevant subject then the job is done. No specific degree or specialist course is a free ticket to F1 - it takes all sorts. If I were you I would try and stick to a mainstream engineering education but at the same time get out there and learn about real motorsport, go to races & race circuits and look at racing cars with bits of metal and tools and hot engines and fuels. Far, far too many applicants who apply to F1 teams wouldnt know a racing car if it ran them over. We look for good graduates but we dont generally take on bookworms who never left the library for 3 years. In F1 you need to be pro-active, energetic, practical and able to communicate with everyone in order to get on. If you happen to be bright and academically good on top of that then great, you should have a bright future.
Most interviews at race teams will be based around practical questions such as showing you 3 metal components and asking you which one is Aluminium, which one is Steel and which one is Titanium. Its incredible how many graduates are clueless with questions like this. Absolutely clueless. Lectures and text books dont teach you this stuff so get out there and learn about it as much as you can.
As you can probably tell, I've got plenty to say on the matter so if you want to know a bit more, come and take a look at my blog at :
http://www.jobinf1.com
or ask me a question via my Twitter account @Work_in_f1
Stick at it as much as you can. Its not the easiest thing to do but these days there are thousands of people working in Formula 1 and opportunities are actually quite common now. Best of luck