Idiot.
There will be no immediate overnight change, and all you Fake-news remoaners have overlooked something rather important, the war on drugs and terrorism means that the powers-that-be already know what is coming into the country with a valid reason, and helped by amazing modern devices called computers, with massive databases.
Some supermarkets have developed their logistics systems into equally powerful tools, which is why Ocados share price has gone from £2 last September, to £11 now. The city teenage scribblers have cottoned on to the value of this distribution system as a result.
The only time we will ever have empty shelves is either during a snow storm, or a bread strike.