Martino,
Regardless of the rights or wrongs of leaving it so late to cancel, remember you are the customer!
Get in contact by phone or email, don’t delay.
Tell them your concerns over the running costs.
Of course they will try and talk you into it, but remain calm, professional but firm.
Worst case they may ask you to pay for the winter pack, but legally I don’t think you have to.
Finally, if all fails, you could ask them to put you down for a new B4 engined model or indeed a T3 (assuming it’s the T4 you have on order)