If you take a worse case scenario, which looks like this:
1) There is import duty (ie a tariff) on EU made cars - you'd only do this to protect your domestic market or favour another source of cars (eg USA). We don't have a domestic maker (yet) but we do have foreign car makers assembling in the UK (whether they stay here when they are currently located here to access the EU market is another question). Tariff of probably something like 2.5%, but by no means guaranteed because as I say, tariffs are about protectionism or favouritism.
2) A nasty "B" crashes the pound and exchange rates against the Euro fall massively. More likely and the results of which could be quite huge. The Euro is currently bumping along at just under a pound, a few points of shift will be almost a percentage point each... could easily add 5-10%.
3) A nasty "B" causes mass damage to the UK economy, killing businesses, raising unemployment which then depresses wages - making already expensive imports even more expensive relatively to UK workers. Mass damage most likely caused by increased costs from #1 and #2.
So #1 is controllable by the UK government, the other 2 are not and are driven by market forces.
On the other hand, to prevent this, a forward thinking government could offer someone like VAG or BMW a tax free deal to build a plant in the UK for the UK market. Its probably not a good idea, but it could happen.
I'm going to avoid saying if any of this is good or bad and just stick to impact analysis
