Sorry you are incorrect in this context - the deposit cannot be retained unless the car has been supplied - simple as that!
No I'm not. I've no idea what legal basis you think your argument is based on but I'm gong to say this once more. The only thing that matters are the terms of the contract. The only legal grounds for having the deposit returned are likely to be:
1) The contract states that it is refundable should the purchaser back out.
2) The contract stated a fixed delivery date which has not been met and there are no terms in the contract that allows reasonable delays outside of the dealer's control.
Contract law also allows buyers to claim compensation if there is a delay in delivery and the scheduled time of delivery was essential (which in this context is a legal term).
By your argument a customer can order a car which is built to their spec and then walk away without penalty the day before the scheduled delivery date. Which is not the case unless allowed by the contract.
One simple way for the OP to get a definitive answer is to contact Citizens Advice and then post back here.