I've been considering doing this for a little while now.
Depending on how I feel in 3 years time after a Chemistry Bsc I may apply to graduate medicine. Most places want a 1st or upper second but the flexibility of the medicine courses is very good. After some quick research some places seem to fast track graduates, others do not, some treat both graduates and undergraduates equally, some do not. I suppose the bottom line is that after a good dcientific degree with some work experience or volunteering, you would stand a very good chance of accessing a graduate medicine programme.