Your desk officer is simply sticking to policy. You need to talk to your AOC and explain your position, point out the stupidity of you having to be reassigned (posted!) for such a short time etc, and tell him you want to go ASAP. Then submit your PVR or whatever it's called this week.
If your deskie comes back with 12 months again, tell him you want an interview with COS Pers, remind him about the Armed Forces Covenant which says you will not be disadvantaged by being a serviceman and point out that you are being prevented from making an orderly transition to civilian life. If you were already a civilian you could change jobs in less than 2 months!
You need to press your case. There have been plenty of individuals over the years who have gone at short notice even with amortisation time remaining. On cost grounds (another good argument) you will actually cost the Treasury less because your pension will be slightly lower for life and they won't have had to cough with your pay and allowances. With SDSR15 in sight, the chances are they will be looking at more redundancies anyway. Lastly, you might want to mention taking MOD to an Employment Tribunal if it all comes to nothing. Crab is right, this has never been properly challenged and I suspect MOD would not want it challenged at all.
Good luck!