The RMP is part of the British Army, so you don't go from the Army into the Military Police. Are you asking about transferring once you've joined up, and how far down the line? If you mean serving with the Rifles for a long spell then joining the RMP you'd need to ask the question once you are in and see what the requirements are at the time. If you mean doing it sooner, you need to speak with your AFCO asap, or again ask when you have joined, but it won't be easy once the Army has invested resources in training you for the Rifles. Why didn't you apply to the RMP initially?