I think that is definitively feasible as long as you have the passion for them.
However, some problems might arise from self-study. You'd better check the new specification to see how practical works. Luckily there is no practical coursework involved for the new spec during AS period which means you can find an exam centre easily and practical part no longer contributes to your overall A-level grade but becomes a separate endorsement along with your A-level. BUT...when you are going to sit whole science A-level, seeking a exam centre which can offer you practical endorsement will be extremely hard, and if you did, that would be very very pricy. That's because the new spec requires you to carry out at least 12 practicals to qualify "practical endorsement " as opposed to 1-2 practical plus written coursework the legacy spec did. I also heard some universities don't need a Pass in practical endorsement if you study a non-science related major,not sure....
Would it be possible that you sit AS exams and get decent AS grade and then convince sixth form to let you start the second year of A-level directly since you haven't got convincing GCSEs.
Apart from the entry to the exams, I don't think self-study these A-level is really a big problem, especially for maths - you just need to practice doing loads of questions either from textbook or past papers, and maths is the easiest subject to self-teach I suppose. For chemistry and physics there are more emphasis on understanding and applying knowledge. Bear in mind there are some people who have achieved high grade by self-teach. All is your dedication, motivation.