This will be a matter fo whether there is a proper written order with all these things clearly stated, or it is something the dealer just said "probably". Technically, in law, a verbal statement is as powerful as a written one, but obviously much harder to prove.
Then there is the fact that the contract will include the small print, the delaer's standard terms & conditions, on the back or another attached sheet. As a single consumer, if these are not balanced, then 'contra-proferentum' would apply, or the unfair terms act. But none of that is any good outside the courtroom.
The best thing to do is to politley, and clearly state your position, that you made clear the existing car had the MOT due, and appeal to the dealer's better nature. Do everything calmly and do not shout or get worked up / frustrated / start threatening legal action as that will only get his back up - hence get you less favourable outcome.
If your old car is going under scrappage and would have failed the MOT, then there is no harm in you explaining how the delay causes you real cost and inconvencience. The courts would see this as grounds for compensation in a contrat case, so it should help to pursuade the dealer to do something to help you out. Be flexible on the type of solution you would accept:
- Maybe, if the white is a solid colour and now he can only get metallic, it would be reasonable for the dealer to absorb the cost difference?
- Maybe the dealer could loan you some alternative car? Even an old trade-in that he may have just got and holds little value to him?
- Maybe if your car would pass the MOT, then the dealer could pay the fee and that would not be so bad?
Hope this helps.