If you want to be a medical doctor, you need to do a medical degree. There's no point doing it if you want to do anything else (although if you start it then decide clinical medicine isn't for you, you will still have lots of options). If your goals are something other than pursuing a career in clinical medicine then just do the shorter degree.
It is possible to apply to medicine as a graduate however it's not a transfer, you would need to complete the entire degree, and it's much more competitive to enter into the accelerated 4 year graduate entry medicine courses (and you would have extremely limited funding for a 5-6 year standard entry medicine course).
Therefore if medicine is your goal, again, you should do the medical degree. You can potentially pursue e.g. biochemistry related research after a medical degree in any event (as one of the numerous possibilities if you decide against continuing in a career in clinical medicine). It's also in fact possible to combine clinical medicine with an academic research career (in some specialties in fact it's not uncommon at all).