If you have something in writing (or on JPA) from AIR Manning saying you can leave at 6 mths, then you can unless you get something else from AIR Manning telling you otherwise. You are employed by the RAF, not your SEngO.
As for aircrew, think most are being held for 12 mths, especially if they are in flying tours. Ground tours/staff appointments might be different depending on how essential you are and how many sickies there are kicking around that could stand in for you. Just depends on circumstances, so no generic answer, sorry!