Well I did A level Chemistry, and I would say that the maths part is about 30-40%,
and keep in mind, the maths is not really that advanced, just using formula's and maybe the occasional algebra here and there. But no differentiation, integration or anything.
Also please, please, please choose the A levels you enjoy first!
Then choose the ones that keep your options open.
I made the mistake of choosing very hard science and maths A levels, not mainly because I enjoyed them, but because I thought it would "prepare" for future "career prospects", I failed terrible and got the lowest grades you could possibly get for all the subjects.
So I'm warning you really, doing something you don't enjoy can come at a very heavy price.