Whatever preparations are made, and I'm sure companies will have some contingency plans, the introduction of customs checks on all European imports will have a huge impact unless the government were to invest multiple billions on the infrastructure to cope with it. Even if they did that, there would be no getting around the increased costs to industry and the consequent price rises for many goods.
They don't do customs checks on all non-EU imports, they just do targeted checks. It wouldn't be much different, particularly with a pre-disposition for acceptance.