I dont believe there is such an act that will support you here - the deposit was paid in anticipation of buying the vehicle.  It was your choice not to proceed, and therefore you theoretically have given up all rights to that deposit.

The fact that they are willing to allow you to transfer the deposit to another vehicle means they are doing you a favour, so I applaud them.

Sorry that this isnt what you wanted to hear, but hopefully you will be able to take some good from it.