When the first three-lane motorways opened, they were also the first three-lane roads of any kind. There was a school of thought that said the left lane was a slow lane and only for lorries. The middle lane was for cars, and the third lane for overtaking. I know, there's nothing like that in the Highway Code, but I suspect that thought process still persists.
I'm tempted to undertake, but if the idiot takes fright as I come alongside and pulls over, he will hit me, and I don't think my explanation will cut much ice with the police. I favour the "move from left lane to right lane, pass the idiot, and back to the left lane" technique (provided it's safe of course). Of course they never get the message. If they were paying attention they wouldn't be in the middle lane. I believe the police call them CLOCs - Centre Lane Only Club.