First, it is not unusual to drop out of school and move to college. No university will hold it against you. They are primarily interested in your academic track record. Yours so far, judging by your GCSE results, is very good. Obviously you now need to pick up again on your academic work (presumably from September!) and achieve some exceptional AS scores (first time round) as these are now entered on the application form, and have a huge bearing on whether you are made a conditional offer. You will also need your referee to be able to confidently forecast A grades in all your A level subjects when you make your application (presumably in September 2010 for 2011 entry). Your extra-curricular activities in which you are currently engaged will also advance your case on your application form when you eventually write a Personal Statement.
 
As for AS subject choices, the key factor is to play to your strengths. People study PPE from all backgrounds - many have done Maths, Physics and hemistry at A level, for example. Maths is certainly key in any subject combination for PPE applicants; indeed, if you are a good mathematician, studying Further Maths as well will very definitely strengthen your case. Many applicants offer Maths, History and Economics - an excellent combination in my view. Government & Politics is another live possibility. English Literature would also be fine. It's important to avoid studying subjects which Admissions Tutors (despite their public utterances) from upon - so beware Business Studies and Sociology, and certainly don't touch Media Studies.
 
The real issue is to develop enthusiasm, even passion, for political, economic and philosophical issues, and to read outside the syllabus. One Oxford college prospectus describes in detail what admissions tutors are looking for. They seek ‘evidence that the candidate is genuinely interested in, and enthused by ideas and issues involved in the subject’. There should also be ‘signs of an ability to focus precisely on topics, to grasp what the problems are in particular areas, and to rigorously analyse and understand the way that different arguments move to resolve these difficulties’. Candidates should be prepared ‘to think for themselves, to try out options, and to examine carefully their own intuitions’. The entry concludes that ‘in short, candidates must demonstrate strong motivation, analytical rigour and intellectual creativity’, and emphasises that the college is ‘able to discriminate between the genuinely talented and the merely well-trained’. To quote one Oxford Admissions Tutor: ‘We want two things from our students – ability and commitment. Tutors are not so much interested in what you know, but how you apply that knowledge, whether you think independently, and can develop arguments. The only thing the students have in common is intellectual energy and drive – there is no such thing as a typical Oxbridge student’. Or to quote the Cambridge prospectus: ‘Who are we looking for? Students who enjoy a challenge.’ ‘We’ll be looking for an indication of your ability to think critically and independently, your willingness to argue logically, but also to keep an open mind to new ideas. You’ll also need self-discipline, motivation and commitment’.
 
So go for it! Your prospects of success will be determined by you.
 
I hope this is helpful.