It is quite legal to "undertake" .. if done correctly.....
The term "undertaking" means actually moving deliberately to pass a car on the left hand side.
If you are travelling in the left lane, and the traffic on your right is moving slower than you, it is permissible to continue without changing lanes .. we do it all the time in traffic queues..... so if you are doing 56 in the left lane, and a plonker in the middle lane is doing 50 .. there is no need to move out... just continue in a straight line and go past.
If you are in the middle lane and you meet such an idiot you must move to the right to pass, as a move to the left is an undertake, not a "traffic moving slower on your right"
So keep to the left and keep a constant speed... but be VERY CAREFUL in case the plonker suddenly sees you and makes a dirty dive left ...

I do this lots with the tin tent on the back as I'm not allowed in the outer lane, and I don't want to sit behind said plonker doing 50 in the middle lane when I can legally do 60 ..
