As I understand it (this is in no way official), around 10 people per year of 150 fail our finals. Most of them, however, don't actually take them - they are pulled aside a few weeks before and "strongly advised" to retake some of their attachments and defer taking the exams. If they persist and go onto finals they can, but at their own risk. This situation may be changing now however - until last year the Cambridge clinical course was only 2 years and 1 term long, so finals were in December and those who either failed/didn't take them, could take them in June and still (hopefully) start work in August in sync with the rest of the country. Now our clinical course is also 3 years and finishes in June, I don't know what the resit situation is - the first lot to take June finals will be doing it this summer so whether they'll have to wait a whole year, or retake in 6 months or something remains to be seen...
From talking to one of last year's finalists, very few people failed the actual medically bits of finals. Some failed Public Health or Ethics & Law, and others failed the communication skills bits (generally, though not exclusively, non-native English speakers). Basically, if you've worked hard enough to get to the end of 6 years, and are not too dangerous to the world, you'll get through.