Hard for some people, less so for others. There is no objective way to measure the difficulty of a subject, and it really does come down to how well your educational background has prepared you for it and how motivated you are to put in the work to keep on top of it. If you got an A in GCSE Maths and are otherwise generally scientifically inclined I think you'll probably find A-level Maths something you can cope with at the very least however.
I would note that for a chemistry degree, there is a moderate amount of maths involved, at least a low (A-level equivalent) level plus some maths at higher levels (although that might be more implicit depending how the relevant areas are taught). So I would recommend taking A-level Maths now if that is something you seriously want to pursue, as you will have much more time and support from your school to learn that content compared to learning it from scratch in a degree through a couple of lectures a week in a lecture theatre of 100+ students.