Ignoring the bickering...
Saying that, how many people fail? Variable. For our 2nd MB exams, most of the 1st year ones have a 10%ish failure rate but you have one resit, which most people pass. In 2nd year the failure rates for some subjects are closer to 30-40%, and some people don't pass the resit. But you'd have to try really hard to fail both times, given that resits are after the summer holidays so you have effectively 3 months extra revision time. I only know a couple of people who've actually left Cambridge - most have transferred onto other courses (usually NatSci)
Do people get thrown out of medical school, if yes, why? Very rarely - you'd have to commit some serious disciplinary infringement, or just not do any work at all/not show up for anything.
What is the drop out rate like, and what are the reasons for dropping out? As has already been described, but it's especially low for Cambridge because they're quite obsessive about keeping you here. A fair few transfer out to other subjects, but I can only think of 2 who have totally left.