You don't need to do both maths and physics - A level physics only uses a few things from A level maths (for example logarithms), and the rest is gcse. The main reason people say you should do both is probably because a lot of courses that specifically require physics also require maths, and because there is a bit of overlap between the two courses.
I think you're overestimating how much unis will care about your subject choices. From what I understand, they don't care all that much what your other subjects are, as long as you have any subjects (and of course, grades) that they require. (And then there's a few subjects that they sometimes don't like, whether that's because it's a language you're fluent in already, it's multiple subjects that overlap too much, or because they don't see it as a very 'good' subject, i.e. critical thinking).