I had a problem leaving the Service some 38 years ago. I decided to leave just prior to my 38 year point to take up a job offer with the airlines. MOD initially said no. I then pointed out that I was serving a contract and not a sentence and that as they had just made a whole bunch of aircrew officers redundant...with just six months notice...they had effectively modified that contract allowing me the same notice period.
It took some time, but in the end they saw sense and agreed, changing I later heard, the notice require for anyone else.
Amusingly I mentioned that I had taken advice from a local solicitor. I was asked, smilingly, if he was up to speed with Service regulations. I played it very gently saying that I could only hope so...but perhaps he had picked up a few tips in his previous job before he became a family solicitor.
...and then broke the news that he had just retired as a Wing Commander in the MOD Directorate of Legal Services!
Keep at them and you will win in the end.