I don't think the jump is too bad for science/maths subjects, I mean C1 is basically GCSE stuff with really basic integration and differentiation at the end. Then unit 1 for AQA chemistry if I remember correctly, I had already done some of the stuff we were doing like moles/titration calculations, covalent/ionic bonding, cracking alkanes etc.
I've done well so far doing maths/sciences and everyone I speak to is like 'omg you must be so clever doing those subjects', but I really couldn't have done essay subjects, I was terrible at them even at GCSE. It just really depends on what you're good at and I think that should be an important factor when choosing A-levels.