As I could not sleep I may add a thought.
In earlier posts myself and others have cast doubt on the legality of retaining the deposit. As it most likely exceeds the actual cost.
To enforce that idea.
Penalty clauses, although often heard about, are NOT enforcible in English law, (at least commercial law). I believe this dealer wishes to use the deposit as a penalty clause.
However. Liquidated damages, (I remember it as liquidated and asserted damages), are. To put it another way, only actual losses can be claimed. So as the prof said a few posts ago, if that can be shown to be £300 then that, perhaps, should be the amount taken. If the dealer might end up with the van as a liability, (highly unlikely at the present time). Then perhaps further charges could be argued. But if the dealer sells the van for more this might negate cost altogether.
However. If damages are not specified in the original contract, (they certainly would be in commercial contracts). It would appear that the dealer should return the deposit.
My thoughts it must be pointed out, are only based on my doing a very interesting and enjoyable high level module in building law (contracts and tort), over 40 years ago. Plus my own experience. However. I would suggest it is sufficient for the OP to not accept the word of the dealer and get the facts established legally.
To expand on what the prof said re buying then selling the van immediately. The easiest answer imo would be to transfer the contract, just admin time. This could be paid for by the OP or the new purchaser. The OP could even advertise it himself, should the dealer agree. And why should they not. Everyone’s a winner. The dealer gets his sale plus a bit of extra admin costs. The new buyer might pay slightly more than the price already agreed but beats the queue. The OP gets out of the contract.
A win, win, win scenario.
John