I don’t believe your GCSE’s will hinder your application significantly.
Yes, Oxford and Cambridge look at GCSE results, but it seems you got the best grades in the right subjects, so I expect they would look favourably on that.
Another deciding factor is entrance exams. Oxford has MAT, which assesses only the year 12 content of A level maths and Cambridge has STEP, which is a whole lot more. Afaik, most people who get an offer for Oxford get around 70% on the MAT, but idk about STEP.
Also look at what each course offers. You should be able to find this out using the respective websites.
GCSE results, quite frankly, are not a good way of deciding which of these unis to go for.