The jump is certainly significant, but if you're an A/A* student you should have little difficulty. The best way to bridge that gap is to do Additional Maths GCSE, and perhaps familiarise yourself with C1 topics before the term.
Any kind of exam technique will need to be overhauled (I remember passing GCSE maths on no sleep or revision and getting As, didn't happen in A-level), and A-levels demand a lot more understanding than GCSEs. You'll probably like Core units, you might not like Statistics units. ExamSolutions and PhysicsAndMathsTutor are invaluable resources for STEM subject self-teaching