A lot of core 1 (1st module in A Level maths) is an extension of A* topics at GCSE, so not overly hard. Core 2 introduces some brand new things which can be pretty hard to start with, though they get a bit better with practise. Maths will certainly help with the science subjects too, so taking them together will allow you to improve with the calculation questions in the sciences. Though I wouldn't recommend it if you don't get an A or A* at GCSE (or the equivalent, if you have the new number system) as it is quite a step up. Anybody who got a B at GCSE dropped it at my school, and now some people who got an A are struggling a little at A2. But if you're dedicated to it, and make sure you save enough time to do loads of past papers, it will be okay!!
If anything, I think physics is a bigger step up than maths is. You do build on GCSE knowledge for physics, but personally I find the questions much more difficult to answer - especially the wordy ones! (I'm usually okay at most of the calculations).