definately do biology, its piss easy if ure prepared to sit down and do a couple of hours noting of the text book of each module. no complex stuff.
chemistry is abit more complex n takes bit more gettin ure head around the stuff, but like biology really if uve got half a brain on u and are prepared to put the work its easy. BTW i found bio and chem gcse completely different from chemistry bio AS, chem AS is more sort of learning the concepts and then applying them, kindof like maths.
ps do physics, its well fun, but harder than chem!!