Ah ok. So we would need to get trade agreements in place for this.
As I've said before, the issue will be non-tariff barriers. If companies have to produce different versions of a product to export to different markets, to meet different regulations, it significantly increases their costs and disadvantages them against local producers.
The beauty of the EU or access to the EEA is that we do not have these with our largest trading partner. This from an article by Robert Peston explains the other non-tariff benefit:
Well the point about being in the customs union is it makes it easier and cheaper for British-based manufacturers to trade with the rest of the EU than any trade deal would deliver.
In the customs union, they can sell their cars, and missiles and electronic chips to other EU countries without incurring tariffs and without having to prove that the content of those goods is largely made in Britain.
Think for a second about why it is incredibly helpful to British makers that they don't have to prove country of origin, as part of the customs union.
Well, in a typical motor car or aircraft wing or chocolate or pharmaceutical there are loads and loads of ingredients and components that are manufactured outside the UK.
Or to put it another way, a great deal of British manufacturing - and a great deal of manufacturing everywhere - is actually the assembly of parts, kit and compounds actually made all over the world.
So the great advantage for a Ford, or a BAE or a Jaguar LandRover of the UK being in the customs union is they can sell their stuff to the rest of the EU without having to prove that the finished item is truly British, rather than a foreign wolf in British clothes.