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