If we left the customs union and the single market this would the outcome Dover and the corresponding continental ports would need massive expansions to handle the necessary customs clearing . it would be so unworkable than trade would essentially collapse overnight . We get around the problem by . 1 . Staying in the single market in the medium term the so called ' interim deal '. 2 . Using the time bought by 1 to negotiate a range of MRAs and custom cooperation agreements with the EU so border checks and delays are not necessary this is actually far more important than tariff free trade . Attempting to trade with only the very basic WTO rules is basically unworkable major importers to the EU such as China and the US have dozens of MRAs and other agreements with the EU and do not rely on WTO terms only . It would be a crisis and is one of the reasons why those advocating an immediate had Brexit are morons .