My background customs & freight but thinking again I don't understand why it seems so hard to understand for so many? Goods arrive at port. They needs to be customs cleared onto the territory: relevant duties & VAT paid (unless have a deferment account) and compliance to market

standards proven either by recognised certification or physical inspection (organic esp to ensure disease free). You need matching infrastructure, systems & experienced staff to meet these demands. However when part of a customs union none of these customs procedures are needed

for goods arriving from your fellow union members. And when part of a single market none of these compliance proofs or checks are needed for goods arriving from your fellow single market members. To move from major demands to none is easy. To move from no demands to major demands

after *decades needing none* needs whole new sets of major supply to meet them, that'll take considerable investment & time to build & train for. Instead UK will be launching itself into the fray with siezed up hardware, semaphore & army of short supply, barely trained conscripts

By its own choice