I assume you mean a medical doctor, when I did A-levels most places wanted chemistry, biology and maths or physics.
Looking at University admissions on the web it seems most Universities want at chemistry, and would like (but not insist) biology or at least one other science.
I remember my school science teacher saying the only A Level you really needed to be a medical doctor was Chemistry. This seemed funny as I always though surely Biology was more important! Maths and Physics are definitely really useful but Chemistry (and sometimes Biology depending on the university) is the only essential one!
Nicci and Jessica are right. Commonly you need chemistry and often biology. After that they quite like you to have either physics or maths (or both). Also, some universities ask you to do a test called a BMAT or a UKCAT – that gives them an idea about what they call critical thinking (not entirely sure what they mean by that!).
It’s worth looking up what individual universities ask for so that you choose the right subjects for the universities you like.