I do that to people who try and undertake me as well, its fun, shows the idiot up infront of his passenger
agree, i had a cop stop me 6 months ago for undertaking. it was a 4 lane road, a car was in the outside lane whilst the inside 3 were clear - annoyed after flashing lights for about 30seconds and the car not gettin the hint, i undertook in the far left lane (2 lanes between us)
Copper stopped me and said 'you cant do that blalablba, here is a ticket' etc etc but ive heard nothing since. Although its illegal, they only charge you if they can prove its dangerous driving, if they see you do it and its not dangerous but 'careless driving' they warn you. (If you continue to do it and get caught i imagine they would eventually charge you)