I find that in these situations, positioning your car towards the right hand side (ie white lines) of the inside lane and not flashing but turning your lights from main to dip to main to dip etc...lets them realise they are silly people without flashing the main beam and noising them up. (he/she/it sees you in their left mirror)
I find that they then move into the inside lane in front of you, allowing you to progress and overtake them before moving on with your journey.
Why they do it i really have no idea. Its just a very oblivious/arrogant/strange past-time that a lot of people like to partake in.
As for the "hand gestures" i used to want to pull them over and introduce them to the "Taylor Made LT" 7 Iron about the head area....but now i just tend to ignore them and give them nothing more than a passing look as if to say "your lucky i am well natured"
but yes your right, theyre facking cants. I have been pulled over on the Edinburgh bypass for undertaking in the past and the real knobbers havent been so much as looked at by the plod for starting their nonsense in the first instance.