Civil registration difficult
As the Viking fleet has no civilian type certificate and the design authority is a company contracted by the MoD moving the type to the civilian register would be problamatical ( but not imposable under Annex 2).
So moving to a civilian registration would inevitably result in further delays when the military system is very close to producing the first servisable aircraft.
As with all these things the hard part is getting the paperwork system agreed and in place, the technical issues are comparatively minor.
As you might expect there are a number of vested interests when it comes to the contract for the Viking fleet recovery and as the MoD has yet to announce the winner of this contract and until this is done all involved are keeping tight lipped for fear of breaking the rules on fairness when it comes to issuing MoD contracts.