The dealer will want to register the car as soon as they can, targets and all that. If you decide to wait and register it in September as a 73 plate, come up with a reason why that isn't because of resale value, you're on holiday, business trip etcetera where it is physically impossible for you to pick it up. Unless you had a specific agreement with the dealer to register in September, you do not have any rights on the matter apart from walking away and losing your deposit.