I don't really get what you said about the high vacancies not meaning there should be more med students. And since so many people apply, as you said there are many to choose from, and many will check all the boxes and still get rejected because of places. I agree given the number of places they have to turn a lot down, but if they did increase the places they wouldn't have to select candidates with rubbish criteria like GCSE grades or ethical debates.
Also then why is the grade requirement for medicine not as immediately high than other subjects? Even for like oxford the requirement is A*AA, even lower than A*A*A for chemistry, but they have a notoriously high bar for GCSE grades, like 10. Why the random skew towards GCSEs instead of A levels, for one thing?