All countries have customs checks, to check that incoming goods conform with regulations, applicable tariffs are paid, etc.

Except countries in the EU, which has a common market that made such checks unnecessary inside of it. Of course there are still customs on the outside EU border.

So if you leave that, and your borders are now not internal market borders anymore but your and the EU's external borders, you're going to do more customs checks than you used to have. In both directions.

They've decided to leave the European Common Market, which was precisely designed to avoid the bureaucracy and reduce the burden of the customs procedures.

There's two sides to every trading relationship. Even if the UK tries to reduce bureaucracy on their side, there's still all the bureaucracy on the side of the other countries they're trading with. And they literally just torpedoed all their trade relationships by leaving a large customs union that allowed them to trade with a large number of important partners with significantly reduced bureaucracy.

I agree with you that bureaucracy here is the problem, and Brexit was the cause of it.

The Brexit slogan was "Taking back control of our borders". Please explain how can you do that without increasing "bureaucracy"?