I think there are two points here:
Firstly, the lesson for the OP is that you do not, ever, ever, make decisions about cars (or, in fact anything else) in this manner. Regardless of what the dealer tells you, the car will still be there tomorrow. And even if it isn't, there will be another one around the corner. Do not - ever - feel pressurised into buying a car. Hopefully this is a lesson learnt for you now.
Secondly, the car has been misrepresented. It doesn't matter what work the dealer may or may not have done once the deposit has been paid. The law does not expect you, the consumer, to know what dates relate to what registration plates. The law does expect the dealer to correctly describe the car in terms of the material facts - and the date of first registration is clearly a material fact. The expectation will be that the dealer will get these matters right because that is the dealers business. It may or may not have been a genuine mistake on his part. That doesn't matter - the emphasis is on him to get it right. So you are well within your rights to walk away from the deal and take the dealer to the small claims court if he refuses to return the deposit. It doesn't matter what his terms of business are. You can argue that if the car had been correctly described then you would never have entered into the contract.
And before anyone thinks I am on the OPs side - actually I'm not. He needs to learn a lesson and hopefully he has. But based on what has been posted it sounds like the dealer is trying to bully the sale to completion and that, IMHO, is wrong. There's plenty of scope for negotiation and compromise and it doesn't sound like the dealer has offered either. Based on the facts as presented I don't believe that the dealer deserves to win.
