Preparation for A levels during GCSEsWatch
Maths you should make sure you're secure on stuff like algebra, (quadratics, algebraic fractions, etc.) trigonometry and coordinate geometry. (intersections of lines, tangents to circles, that sort of thing)
I'm not that familiar with the new GCSE physics spec (did the old one), but reading the Edexcel one there seems to be quite a lot of overlap. If you're confident with most of GCSE physics, you should be good to go into A-level. (indeed you'll spend quite a bit of the AS recapping what you already know) I came in with pretty weak understanding of the GCSE, had pretty much blagged through the exams, but managed to pick it up.