Imagine that you have four people each trading exactly the same amount with each other. Now imagine drawing a line around one of the people and calling him a country, with no change in what he trades. You'll find that the one person exports 100% of his GDP while everyone else exports 25% of their GDP.
All you've done is made an argument about any tariffs anywhere, for any purpose. It' naturally falls out from the way small and large countries work; it has nothing to do specifically with the UK and Europe.