I applied for medicine this year and should go to med school this sept as long as I make the grades (hopefully haha).
I took three sciences and maths at AS and dropped physics for A2, almost all UK medical schools require Chemistry A2, about half require Biology A2.
Most of them say you need at least two sciences including chemistry for A2 level so you'd need to take A2 chemistry and either biology, physics (or maths in some places). Just be aware that psychology doesn't count as a 'science' (in any medical schools I looked into at least). So as long as you take two sciences (i would recommend chemistry and biology to give you the most options), you can choose whichever third subject you want. Also, although places like Cambridge say they like 3 sciences, UCL actually say they prefer you to take a third subject which isn't a science so it varies between universities.