Your best bet is to go ahead and apply through the CoC to begin the commissioning process. When you go to Pre RCB they will advise you if they think you are a suitable candidate for the POD Cse (it does stand for the Potential Officer Development Cse).
If they give you the option of going take it. Not only will improve your chances of passing RCB you will also learn a number of things that will help you for the rest of your career. In addition to that, if your cse is anything like mine, you will also drink a horrendous amount of alcohol and make friends that you will have for the rest of your life.