If you are medically discharged you may be entitled to an immediately payable pension know as a Service Invaliding Pension. The precise amount will be dictated by your length of service and rank at time of discharge.
You will not be entitled to a SIP if you were discharged before the end of your basic training or if you take PVR.
The Group can put you in touch with the expert Pensions Advisors at the RBL if you require more detailed information. APC will write to you before your discharge to advise of your likely level of pension.