I wouldn’t worry about leaving the car until after March 1st, just tell the dealer you want to collect it after then with a “20” plate on it and if they start to get funny then tell them you will not accept delivery if it doesn’t have the later registration, thinking about it you are having your own plate put on so that’s a little moot but stick to your guns and tell motability you won’t be available to collect the car until after the 1st of March but you don’t want the dealer pre registering the car as it will put another owner on the car.
I did this once with one of mine that the dealer was trying to get his figures up with, refused to enter pin number, ended up with the car registered as a “65” plate and not a “15”