The seller is rightly furious as they now have to pay a non refundable listing fee of £14.99 for a car auction and to pay it again if they relist the car. So, it's costing them a lot, and they are in their right not to cancel at your request.
They may open a non payment case / cancellation after 4 days, this gives your account an unpaid item strike. If you gain another you will not be able to buy from the majority of sellers for a full 12 months.
Perhaps change your password on your eBay account and log out completely when not in use, so something like this doesn't happen again.
@big.wheel