Chemistry is the only required subject for the vast majority of Medical Schools. The problem solving skills learned in the subject translate nicely into medicine.
It requires both a solid theoretical and practical understanding to succeed in it. Where as Maths is largely theory (with practical applications) and Physics leans more the theory side at A level.
Chemistry also overlaps directly in places. Medicine also makes use of the 'mole' quite a bit (millimoles), MRI is essentially applied NMR and you won't understand how blood pH is maintained at a constant if you don't understand the Carbonic acid buffer system, making use of Le Chatelier's principle. Most of the biochemistry and enzymology in medicine is applied Organic Chemistry and kinetics that are learned at A level.
It has transferable skills, is very difficult and overlaps nicely with Medicine.
Biology is usually the other subject to take at A level, however it's an easy subject to pick up, where as Chemistry is not. Also involves a lot more memorisation, instead of critical thinking.
Chemistry separates the good from the average at A level.