My two cence is;
Don't do too many, five is excessive. I met a guy who got into Imperial after doing 6 A levels all grade As and his advice to us at the beginning of our A levels is it puts unnecessary pressure and stress and made no significant difference. Sure if you want to be ambitious give it a go, you can always drop one or two. I am doing Maths, Chem, Biology, Polish and Theology as an As, dropped out of French A level...
Physics is very similar to Pure maths and Mechanics, and that stuff is no piece of cake. Universities will look at your predictions for your 3 main subjects and one as, they look at your extra curricular work... Your going to have trouble to handle everything if you pile up that much work, then extra curricular work, work experience you wont get a break. Sixth form should also be fun not stressful.
Enjoy it and work hard, don't over work yourself as medicine is no piece of cake either another what 5-6 years of extremely demanding work.
Maybe it's because I just have a bit of a laxed outlook on things but trust me it's a huge step up from GCSEs and the workload is going to be much bigger for 4 subjects let alone 5.