I believe the drop out rate for medicine is actually relatively low compared with other courses, and certainly compared with other healthcare courses like nursing. Those who do drop out usually do so because they either failed an exam + resit, or because they decided medicine wasn't for them. The number who quit to become pop stars/actors is vanishingly small, but I guess they still come under "decided medicine wasn't for them."
Although individual universities will have a few idiosyncracies, the general rule is that if you fail an exam you get 1 resit (some places make you resit ALL your exams, most just the one(s) you failed). If you fail that, with no extenuating circumstances, then you are out. Sometimes you may be allowed to appeal and either get a 2nd resit or repeat a year, but this isn't always allowed. And some places, like Cambridge, will pull you out before exams if they/you think you will do really badly, and get you to repeat that year anyway.
From my cohort I can only think of a handful who didn't complete due to failing. One or two left very early on, and a couple didn't continue to clinical school, but the vast majority got through one way or another.