would be prepared to leave all their pensions, benefits future promotions etc
I suspect there would either be a transfer of benefits to iScot or, more likely, pensions accrued would be frozen and paid by the rUK as and when it is due (in the same way that pensions are paid to expats).
As for promotions, many who have been 'passed over' in the current UK Armed Forces may see transferring across to a new ScAF, ScArmy, ScNavy as a way to re-invigorate their career and may actually move up the rank scale on transfer!