You have to do biology and chemistry.
To keep open the option of EVERY medical school you need to do one other of physics or maths - if you think you can't handle four A levels, do maths (as physics without maths will be very difficult, particularly if you're not amazing at maths already).
Don't do psychology unless you're already doing biology, chemistry and maths - psychology is considered a humanity, not a science.
I know doing four A levels may sound like a lot - but to be brutally honest, so is the workload of a doctor. If you think you'd "crumble" under four subjects, maybe reconsider the realities of this career (I mean this in the nicest way possible, just want to make sure you're sure about this!).