1. don't presume that you'll be fine just because you got A* at GCSE or are predicted it. Many students end up gettings Ds and lower at AS as they just can't grasp the subject any more and find it a big step up- depends on you and your ability really. If you're good at learning stuff Bio should be ok if you put the hours in. Therefore, don't presume you'll do Bio and Chem to full A-level, what if you got a U in Chem (just saying worse case scenerio but im sure you wont if you work)
2. Do what you'll enjoy. As long as you have Bio and Chem you'll be fine. Ask current students about the subject, have a look at textbooks, past papers etc
3. Don't be to sure of becoming a GP, many people change their mind at med school. I'm in sixth form and was set on Dermatology, but work experience made me realise how many other interesting fields there are
4. Best of luck
any idea where you might apply? < i know its early days, and you're likely to change your mind many times.
UCL like a language at A-level ( but they only encourage it , if you enjoy and can do well in it) Peninsula like a humanity ( if you know you can do well)
Cambridge (like Maths or physics)