I understand that the latest Sqn Ldr PA Spine ToS has a clause that states:
'...employment will be limited to OF3 flying or flying related staff appointments'.
This is far from the bias towards flying appointments there was in the past. It would require a critical shortage in flying appointments before an SO2 was released to a down-borne OF2 post. There is nothing that prevents being appointed to an OF3 flying post, just that there is usually no difficulty in filling these posts with career stream OF3s. The amplification attached to the RoS goes on to state:
'…the majority/all of your remaining service will be in staff appointments.'