Are you longest in post ? If not then how can they justify posting you as a Cpl, I would think it's an idle threat and they are ****ed with you not accepting what they have offered.
To me you are very lucky you haven't got a really family wrecking posting, the first whatever number on the PSL ended up gong to 90SU what a reward for their efforts. Fancy going to Eastern Europe or Middle East for 6 months at a time, thought not.
Promotion to Sgt is damn hard to get (taken me 20) and the initial posting rarely works in your favour, they have dangled the carrot and if you choose not to take it there are a few hundred hungry people who will unfortunately.
I obviously don't know your circumstances, but to be brutally honest unless you have a rock solid reason why getting posted to ??? will ruin yours and your families life then they might just think you are being a spoilt brat not getting what you want.
If you have a preferential reason why you are in situ then surely this will apply to Sgt as well as Cpl
For me it is a family wrecking posting - I'll be away Sunday to Friday as they work a set 8-5 shift and not the flexible working that a section say PJHQ work (start late on a monday/finish early on a friday as long as you make up the hours midweek). I have a daughter starting her GCSE's plus other financial commitments mean that my family can't/won't follow me.
As for longest in post - I have been at Waddo for some time, but I am aware of other Cpl's who have been in the county a lot longer than me, they've moved between stations and sections.
As for its taken you 20 years - I'm slightly longer than you (the offered promotion date is a day before my enlistment date, 22 years this year). I have the added issue that I haven't been offered LOS 30 and probably won't due to my Med Cat - I've been offered an extra 12 months on top of the 2 years I've just signed as a Cpl. So if I did accept it then it'll pretty much mean that I'll be stuck down there till the end, I have previous experience of asking for a move in my last 2 years, even as a LTOD, and got pretty much shot down with the phrase "the gaining unit won't get a full tour out of you - so no".
As for going to the Middle East/Europe - I've never turned down any form of OOA, I would say that when it comes to OOA's you would struggle to find anyone in the trade that has done more than me, and certainly involved in current Ops. Jokingly I'd probably see more of the family if I did deploy on an OOA that being posted to Northwood. The Sqn I'm currently on has identified the need for a 2nd Sgt, paperwork submitted and approved by FHQ at the end of Jun 16. I have all the qualifications and Operational experience to fill it, the Sqn want me in it, Desk has turned around and said its Northwood or Nothing. I have chosen the latter. The Desk are making out that I'm the only person, either from the current pool of Sgt's or those of this years PSL who can fill this job.
It's purely because of this that I'm being threatened with a posting.