>It's not so much the tariffs as the customs clearance per se. Everyone seems to have forgotten that transporting goods across a border has VAT implications, and if you don't check and aren't part of the same VAT area there's a huge opportunity for "carousel fraud".

That would give the EU reason to block imports from the UK. It doesn't give much of a reason for the UK to block imports from the EU. So there's no reason to expect that the UK will starve or Amazon will shut down. The only way for that to happen is for the UK to shoot themselves in the foot (again).

Exports, of course, are a whole different story and that's where the risk lies. General economic damage from increased friction in exporting to the EU.