I have my private plates on my car which I took delivery of in mid May.
‘My plates were on a retention certificate and emailed a copy a couple of days before collection. He registered the car with my number and I gave him the certificate when I collected the car.
i had no need to register Motobility as the owner and when I go to change the car, I can move number to next one as I still own it.
Bottom line is that it’s easy to do, can be done without Re-registering the car and you dealer can do it and if he says otherwise, it’s because he can’t be bothered or doesn’t know the rules.