From a debate in the House of Lords:
Lord Davies of Oldham: My Lords, overtaking on the inside is against the law. Significant stretches of our motorways are under surveillance. The driver of a vehicle who deliberately comes up behind another vehicle in a middle or outside lane and then swerves inside to overtake could easily be charged with careless driving.
Lord Elton: My Lords, will the noble Lord kindly write to me�he will not have the information at his fingertips�to say when it was made illegal to overtake on the inside? Many of us have been saying that it should be made illegal but were told that it would not be done.
Lord Davies of Oldham: My Lords, the actual manoeuvre of one vehicle going past another in the inside lane because there might be a slow-down in the outside lane, and the inside lane is continuing to move, is not illegal. Deliberately setting out to pass a vehicle by moving from one lane to another to go inside it and to go past leaves open the possibility that such a manoeuvre appears to be careless driving, and it could be subject to such a charge.
http://www.publications.parliament.uk/pa/ld200 304/ldhansrd/vo040209/text/40209-03.htm