Making a case
In my opinion, you are focussing on totally the wrong thing. There is no real issue about amortizing training costs etc, the RAF seems to be able to pretty much shuffle a persons terms of Service to suit their needs anyway. The issue is aptitude. A civilian PPL shows no relevant aptitude, beyond the first few weeks of basic flying training. Civilian flying training focusses on keeping you safe while flying around in a Cessna. From day 1 military flying training is focussed on developing the skills you need to fly a fast jet operationally (in incremental steps of course!). All a PPL will give you is a little more awareness and lots of bad habits.
The age limit is in place because the standard bell curve of aptitude falls away sharply in a person's late 20's. The only 'case' you can make is that you have the aptitude of someone below the age limit. To do that, you need to get yourself down to OASC under the pretence of wanting any branch, sit the tests, ace the tests and then ask for a Pilot place. Nothing else is likely to help. If no-one is going to let you apply for Pilot, put an application in for any Officer branch, then try to change when you have proved your 'case' by demonstrating the aptitudes down at OASC.