Hi
I think it would probably be best if you contacted Oxford and any other universities you'd be interested in applying to, and ask for their opinion on your situation.
If I were in your situation, I would probably re-sit year 11 next year so that you have a full set of GCSE's, and then do 4 A levels over the following 2 years, so that you are able to go to uni at 19 with a full set of grades. Although it would involve re-sitting year 11, Oxford sift through applications by GCSE grades, so only having 6 would likely set you at a disadvantage. This way Oxford would be able to see you have the ability to get high grades over many subjects at GCSE, and not just be taking your word for it, like they would be in options 1 & 2.
I am currently re-sitting year 12 from home, and its a lot of work maintaining discipline. Oxford expect applicants to have all A*/A grades, so you'll have to be honest with yourself whether you think you will be able to manage 8 GCSE's alongside a whole maths A level, and get straight As. From personal experience, A Level Maths seems easy to start with, but without a tutor you may find yourself out of depth by C3/ C4.
Ultimately though do what the universities say they would prefer, as at the end of the day they're the ones judging you. Whatever you choose, I hope it works out well!