In anticipation of coming into some money I went to a garage, chose a car and was asked to leave a deposit of £200. Four days later I discovered I was to receive much less than I’d thought, so I cancelled my order. I guessed there would be a penalty to pay, so I wrote suggesting £100. However they refuse to discuss it.
You entered into a contract. If you don’t complete your side of the bargain the seller is entitled to compensation. Where there is an available market for the goods, ie the garage has plenty of other customers to sell the car to, they are only entitled to a nominal amount. But they could easily argue that they were entitled to £200.