I assume you mean physics and additional maths at GCSE? Taking A level maths is definitely achievable. In all honesty the core content of it is very similar to GCSE and some modules are (for the most part) much easier than others.
Physics however... you're really expected to know a lot of stuff from GCSE, so if you didn't do any physics at all at GCSE you'll need to be prepared to spend a lot of time doing extra work to catch up. It's also one of the most difficult A levels, so make sure it's really what you want to do before picking it.