Fortunately I only have to run to the UK (I work for a Spanish firm) once every couple of months - on the latest trip i loaded for Gibraltar which meant visiting the FCC in Dover.
All i can say about the experience is who the hell do they think they are? I have never met such a bunch of ignorant sods in all my life - it was bad enough being English and able to speak the language, but the way some of the other drivers were treated was disgusting.
It seems that the attitude is “You have to come here to clear, so we can and will mess you about as much as possible”. What a bunch of complete tossers!