I think it probably is a bit early to think...
What do you mean by MB/MDs? Anyone can do research, not just PhD students.
I'd recommend getting into medical school first, and arranging research projects early on - i.e. the first summer or within 1st year, to get a taste. Most Unis have research societies that can help you do this, or it will be a compulsory part of the course. This will give you an idea of the realities research, and give you a chance to make contacts.
If you still decided to become an academic doctor, some students who show excellent academic prowess can do a PhD during their medical degree, I know one girl who did. So instead of doing a normal intercalated degree, you take 3 years out to study for a PhD and then finish medicine off.
I'm not quite sure what you mean about doing an MB/PhD - you can't study for a medical degree and PhD at the same time, except in very exceptional circumstances as mentioned above.
The easier route would be to go to med school, do loads of projects, intercalate, do the academic foundation programme and go from there. If you do a PhD during your clinical training you get a nice bump to your wage.
Hope this helps.