Websites generally only take your revision to a certain level, GCSE B standard, which probably isn't good enough. The CGP guides aren't as fantastic as everybody makes them out to be, but they're OK for Sciences. Also, the Letts study guides are pretty good. If you download the sylabusses off on of the exam board's websites (for the Sciences they're all fairly similar) and have a look at what you need to learn. But if you're studying for medicine, you've been studying the sciences (well, maybe not physics) so you should know beyond the GCSE level, although some stuff probably won't appear at A-Level or equivalent. In reguards to the Maths, half the stuff that comes up at GCSE is a joke, and the other half difficult, but you're probably doing maths so I'd say you'll be OK.