The only one I can speak for is maths - maths A Level is like the harder end of GCSE in AS year though statistics is hard if that's the optional extra module your school makes you take. I am just doing M1 (Mechanics) now and it's not too bad, much easier. Basically though if you pay attention at school and do a load of practice, like every past paper twice, and constantly keep working at papers and practice questions then you'll be fine. That's the only way you can revise for maths really, past paper, both for your exam board and I can recommend Solomon papers which are designed to test you a little more. However maths is certainly easier than Chemistry so you may want to seek advice on that!