As many EU truckers get paid per kilometre any delays with new red tape , on a return trip back to mainland Europe is a big set back to their earnings , hence they choose other European destinations.
There is also an issue of reduced number of pickups and drop offs in the UK.
In the past they could do number of drop offs and pick ups on the way to a place, where they collected a final load to the continent .
For example they could have a load from the continent to say Plymouth but return load was from say from Newcastle .
They could do number of collections on the way ( say from Plymouth to Brum , layer Brum to Leeds , Leeds to York , York to Newcastle ).
I think ( but I am not sure 100%) that stopped or has been vastly reduced .
What it means is that there is less opportunity to get kilometres in, plus there are more empty runs to do , making the continental transporting companies charging more the deliveries to UK consumers.
But hey that is what brexiters voted for .