If you didn't already have your A Level grades, GCSE grades would be taken in to account too, particularly at Oxford (Cambridge make up for this by generally requiring slightly higher A-Level grades than Oxford for many subjects).
However, as you already have 3 A-Level grades at the very highest level, your competition is going to be how well you perform in an interview - and against the more than half of eventual entrants who, like you in A-Level terms, have no more than the minimum number of required A-Levels. Cambridge do tend to interview more but I suspect that Oxford will interview you because of your GCSES (if not because you're self taught too).
Anecdotally, from stories I've read, Oxford seem to have more frequency of rejecting people with perfect A Level grades. I feel that your best chance is at Cambridge who appreciate late bloomers more (e.g. Cambridge has 3 colleges for mature students, Oxford one) but if you like Oxford more apply there. Further Maths will be highly useful, nearly mandatory, at both universities to gain acceptance for that course.