Why is there such a difference in European full load export and import prices?

The simple answer is supply and demand.

As we import more from the EU than we export to it, the demand for trailers to take goods from the EU to the UK is greater than demand for trailers from UK to EU. The price of export trailers has increased particularly since the financial crash in 2008 as the Pound weakened against the Euro causing an increase in exports to the EU. The increase in demand for trailers to go to the EU from the UK caused an increase in export rates across the board