It's unusual, it's usually under the counter (SGUL was unusual last year) and it's often only for applicants previously rejected....but 1/20 entrants isn't a tiny amount
Very rare, usually they are looking for people with high grades and good experience/commitment to the profession.
SGUL was very rare in going into clearing this year, but they kind of shot themselves in the foot as they had over 1000 calls on results day and more after that regarding clearing, so much so that it was more competative to get into SGUL through clearing than normal entry.
Most people get into medicine through clearing lists, this is when medical schools have interviewed a candidate and they have narrowly missed the offer and hence they are put on a waiting list just in case spaces become availible if people miss their grades. This happens a lot.