When I was at college we had a lecturer who had obtained his first degree in mathematics and then gone on to get his master's and doctor's degrees. From there he came to our college as a lecturer but he quickly realised (by the end of the first term) that this wasn't the career he really wanted. He applied for medical school and was told that if he could study the A-level Biology course, in his own time, in two terms and get an A-grade in the following summer's examinations, he'd be accepted. I think that he actually got a B grade (which is still pretty good after a few months personal study!) but he was still accepted for the course.
All of that was many years ago but, combined with the responses above, it suggests that you might still be accepted for training.
If you click the relevant link here (GP, Hospital or Surgeon), you'll find more information about entry into medicine:
http://www.learndirect-advice.co.uk/helpwithyo urcareer/jobprofiles/category28/
Chris