Not being funny, but while it would be lovely for those who want to leave to get redundancy, as they will probably go anyway, what they should have done is take no notice of preferance at all.
The only good thing about redundancy is the chance to get rid of dross.
The dross will never volunteer because most of them know they are dross and may never get such a good gig again, and the good guys who want to leave have no complaint since they new the deal they signed up to.
The guys who want to go will leave anyway, albeit annoyed, but at least you have chopped some dead wood.