Surely this is simple,
Sqn Ldr & above: must complete return of service AND notify (by whatever devious means) of your intention to take your NEW option, as extended service to 55 is guaranteed.
Flt Lt: Has no guaranteed service beyond 16/38 pt unless accepts PA. PA has to be accepted by a certain date (1 yr to go). Legally, the offer of PA MUST be accepted before further service is given - a nil return equals no PA. Unless they say differently on the offer letter (but I suspect this wouldn't hold up at a tribunal!?).