Having completed the BSc Management at Imperial I can confirm, as others have also, that entering IB is perfectly feasible - a few members of my class did so.
You can always have a year out of medicine doing IB, which is more common at medical schools with a preclinical/clinical division.
Whether you will survive 6 years of medical school knowing you want to do IB is questionable (but possible). If you are committed to medicine though, go for it. It's a versatile degree that can be used elsewhere - these options can easily be explored for a year or two.