I had this issue with VW a few cars ago. I ordered the car with the expectation that it would be delivered on Sept 1st putting it on a 19 plate but the dealership wanted to register it as an 18 plate but still give it to me on Sept 1st. I told them to go away and cancelled the order.
My new car arrives this week at the dealer and I have arranged to pick it up next week meaning it will be a new 25 plate.
My advice is to stand your ground and tell them that you want the car as a 25 plate or you will cancel the order. They are trying to hit targets is all.