If you ordered in March, you dodged the price rise on April 4th. If you make changes now, you'll be paying post rise prices for the car as a whole, in effect its likely to be a whole new order - that'll cost you a fair bit!
Also, it should mean nothing to the dealership for you to defer registration and collection to get the new plate if the car was going to be ready mid-August.
Yes a good point about the price rise early April. And not ally when cars arrive august time they have no issue folding till Sept as you say