You have a defined benefit pension, which is a pension based on your length of RM service and your pay when you left.
RM pensions accrued up to 31st March 2012 are now funded by the government and come under the RM Statutory Pension Scheme.
You can't cash it in I'm afraid – government rules!
The only option is to take it as a pension for the rest of your life, or else transfer it into another defined benefit scheme, which you would have to be an active employee member of.
Take a look at the RMSPS website for more info: https://www.royalmailsps.co.uk/