Need to clarify one or two things for you.
The RM have no doctors - all their MO's are on secondment from the RN. A couple of people I know joined the RMR as students, earned their beret & then joined the RN on cadetship.
The military have no medical schools. At the moment, you will have to get into medical school (which will require ABB at least at A-Level, one of which used to have to be chemistry) - once you are there, it may be possible to get a cadetship, but (IIRC - and I was not a cadet) they won't give you that until 3rd year (of 5)
I should also point out, with a heavy heart, that it is a particularly bad time to be considering medicine in the UK - workforce planning has been more or less totally cocked up, and at the moment there are fewer jobs to go around than there are junior doctors. This is a situation that is unlikely to change in the near future, as the medical school expansion years have not yet come through. There is a distinct air of despondency amoung my NHS colleagues, many of whom are considering emigrating to Australia/NZ/the US as the situation shows no sign of impending improvement.
The upshot of this is that the services are more or less in a position to pick who they want - so there is no guarantee you would get a cadetship in any case - and if that happened, would you want to be in a profession with significant unemployment?
Finally, the AMS has decided that all Regimental Medical Officers must be qualified GP's - this will effectively end GDMO jobs. At the moment, the RN is not intending to follow suit, but there is no guarantee that this will not change.
Sorry to be negative, and if you have considered all this, I would still encourage you to go for it (as there are few jobs as varied & challenging) but I would recommend you think hard about the downsides - is there any way you can speak to someone who is a military MO or NHS SHO?
APN