Well, the requirements for Oxford are Chemistry and one other science, whether that be Physics, Maths or Biology.
However, UCL states that 'Chemistry and Biology must be studied at A-level' so, I think Chemistry and Biology would be the best choice as some Universities require Biology and all of them require Chemistry, so it opens your options.
Most applicants have 3 sciences, so maths, chem, bio and physics - but it isn't a requirement. I know many people who aim to study medicine and their 3rd option is something they are good at/enjoy, considering the entry requirements are so high. English Literature, History, Psychology and Economics are frequent choices I see among my friends who take Chemistry and Biology as well.
Cambridge states 'although some Colleges consider applicants offering only two science/mathematics subjects at A Level (or equivalent), please note that the success rate of such applicants is much lower' so perhaps it would put you at an advantage to do 3 sciences, but it isn't necessary.