There is also a QR under which you can leave up to three months early if you have a firm job offer and your current boss is happy to gap your post for those three months. The important thing is that you keep all your entitlements. I used this QR to leave early with a firm job offer and the only effect was a very slight reduction in pension for the three months pro-rata over LOS.
There are ways and means to make this work and I would suggest your best bet is to go see the Chf Clk in Handbrake House - he/she should know all the options open to you and will probably have a better grip on the possibilities than the OC - get PSF onside and he they well get the Desk onside for you and after that life (can) become a lot easier.
Good luck - I hope it all works out.