If you cancel now you'll lose your deposit, and the vehicle, whether or not it would have beaten the Brexit deadline and whether or not there's anything extra to pay.
You're in no worse a position if you hold on for a few weeks/months and then cancel if things aren't going the way you want.
With regard to your contract, they're not going to waive the part that refers to variation in tax because that covers any changes in VAT at the time the contract is completed.
The consumer doesn't pay the import duty, the importer does. The dealer can't charge you import duty, they can only adjust the price of the vehicle. I'm pretty sure the contract works in your favour in this instance.
(Let's just chill for a bit and see what Tom + Breeze digs up)