This is also a proportion of students who take A levels which is already filtering down to about 38% of the population in that age group (766k 18 year olds in U.K. so about 643k in England of which about 250k take at least one A level.)

So in a normal year that 12.3% of A level students getting 3 A’s is only 4.8% of the age cohort.

Edit: I should add that medicine is about the hardest subject to get into in U.K. (other than vet med which has so few places) and one of the only ones where they expect you to demonstrate suitability beyond academic performance, e.g. work experience in a caring setting. (Source: shared a house with a med student in undergrad.)