You do not actually need to have a registration document to get plates.
What you DO need is to provide documentary proof that you are entitled to have the number plates made.
Sadly may places, including some Halfords, do not understand this and insist on a VRD.
When I bought my current van I did not have a VRD as the car was new so I took the sales receipt for it showing details of me, the dealer and the car.
I also suggested to the caravan dealer that he might like to ring the car dealership who would confirm ownership.
Photocopies were made and the new number plates duly made and affixed to the caravan.
Steve