The rules are in place due to cost, paperwork takes time, to save that time and any questions about your private plate do it yourself. Order a car let it get registered, ring up motability 24 hours after asking for logbook reference number, if they do ask (mine is related as it’s all my name) but it can be personal for other reasons including it was my dads. The go on dvla website and assign the plate yourself. Then when next leas comes round call them a week before hand over and ask for reference again, the put plate on retention then (you will get a digital copy) you will also have a set of plates from when you picks up the car to put back on.