All 27 countries have to agree on the terms of our new arrangement, I believe? I assume every country will want to punish us in some way, so that no other country amongst the 27 is tempted to do the same and destroy the whole thing for them. It would be pretty ridiculous if we walk away with better trading terms and don't have to accept the 'bad' bits we're walking away from. I assume that there'll be countries among the 27 who see opportunities to snatch industries from us who'll throw spanners in the works somewhere along the line. Banks are already carrying out due diligence exercises about moving to Paris or Frankfurt, running marketing campaigns to try to be the destination of choice for those in the financial industry
Assume that in the worst case scenario, most car manufacturers leave the UK and we can only buy from foreign manufacturers or niche UK ones that have remained here. We'll have to buy more expensive imported cars (or be left with a limited selection of UK produced ones) and the EU countries won't care about not being able to buy our cars.