Hi Evlad!
Do you already own the personal plate you wish to put on the new car?
If you do, and you are the nominee on the Retention Certificate you just need to give the certificate to your supplying dealer and they will register the car with your personal plate. This way it will come on the car at delivery.
When you then take the plates off the car, a new registration will be allocated. In Northern Ireland this will just be whatever the latest plates are.