A-level physics will be of some use in the ATPL theory exams, the maths involved is no harder than GCSE level. Basic arithmetic, algebra, graph interpretation and a bit of trig, but certainly no calculus (which makes up the bulk of A-level maths).
I found A-Level physics very easy (around 98% overall in the mocks and no doubt similar in the real thing), but maths was a massive learning curve after GCSE. So if you're going to do just one of them, do Physics.
The hardest thing about the ATPL's though is the sheer volume of material to learn.