Keep any conversations polite, you’re more likely to get somewhere.
And although you’re technically right that the car is already registered to you, it’s not your responsibility until you enter your pin. The dealer will know this and won’t want to be left with a pre registered car, so if you’re prepared to meet halfway with him, there’s a good chance he’ll do it.
Good luck, and as Glos Guy said, don’t let this take the shine off new car day. I picked mine up on Tuesday and love it, so while your issue is frustrating, you’ll soon move on once you realise what a great car it is.