I didn't have to do any of that on both my GTIs. I simply handed over the retention certificate to my dealer 1 week before handover and the car came straight with my private plate. Upon returning the car back, a few days before that just do the online retention process with DVLA, pay the retention fee and they will assign a random plate for the car's age. You then take off your private plates off the car and put the random ones and then you can assign the private plate to another car.