The two hardest courses to get onto at university are medicine and veterinary science so, unless you're in the top 1% academically, you might as well forget it anyway. You'd need 'A' grades, at A-level, in at least 3 science-based subjects, with Maths, Biology and Chemistry probably being the subjects which are most likely to impress universities. However even three 'A' grades are often not good enough to study medicine; a 4th A-level pass (at 'A' or 'B' level) is sometimes required.
However, if you really shine at science, then joining the St John's Ambulance Brigade might be a good first step. Sooner or later you'll encounter plenty of blood!