Just let them register your car so you get plates added, then transfer to your private plate through your Motability account online, its that simple. If you do it this way then you will have plates to put back on when you place it back onto retention if you decide to put it one another vehicle or leave the scheme. I do it like this every year and it takes 10 min plus 10 min to swap the plates over on the drive.
Definitely best plan and its so very easy to do. I guess the small challenge for some will be the act of actually changing the plates on the car, but its a 5 min job if the dealership were to do it (or local garage)