Manning are never going to offer an extension of service at the 16/38 point. They'd rather you took PAS which is win-win for them - they keep an instructor whilst at the same time making it harder for you to get out.
The way the RAF see it is they offered you PAS, you said "no thanks, I'm off ha ha!", economic climate changes, hold-pool closes down, you see empty bank account looming so duck back into the RAF to accept the PAS offer at the final bell. The terms of PAS are crystal clear - serve to 55 or PVR to get out. There is no Catch 22. The RAF can't just sign pilots on for random lengths of time just to tide said pilot over until the airlines pick up - that's of no use to them at all. That you already passed selection into BA is equally irrelevant. If after 12 months in the pool you have no course date, you have to redo selection, regardless of where you've been or who you are.
I took PAS because I thought I'd happily serve until 55+ in a job I enjoyed. Things change, job suddenly becomes dire, rubber dickings get handed out, other options get investigated. I now have to PVR to get out, as per my terms of service - I have no issue with that. Ultimately Manning are going to do everything they can to protect their "investments". If they want to keep fabulously experienced, wise and able PAS instructors like me then they are going to do whatever they can to stop me going (NB. "stop me going" as opposed to "keep me in" - a subtle yet traditionally RAF-esque difference). They aren't going to sanction an airline selection procedure that makes it easy to swerve my "contractual commitment" to age 55 - and who can blame them?