As Hype en Ecosse has pointed out, medicine is not the best preparation if you want to be a research scientist, there is little in the way of the sort of teaching (let alone lab work) needed, which is why people who have done a medicine degree and then later want to do research do an MRes then a PhD.
Most people who do this are wanting to also work as a doctor, and most frequently are interested in clinical research rather than basic science research. The combined MB/PhD seems to also be aimed at these types of people.
A medical degree would not prevent you from doing the type of research you describe, but as it does not include the necessary training you are looking at even more years of training (and spending more money on it) on top of the already long MBBS in order to get where you want to be. It doesn't seem like the best option for you imho.
On the other hand you do already have a medicine offer; have you changed your mind about careers since you applied?