Doublechocolatetiffin The HC contains rules and advisory notes, it also does not refer to everything and is not the law. Nevertheless, as many motoring specialists have noted, nowhere in the HC does it say that passing on the inside is illegal nor is there any rule against it, and is therefore permissable. As Honest John says, "it makes far more sense and is far safer than attempting to pass a slower moving vehicle on the offside. You would have to be unhinged to drive from lane one to lane five to pass a slow moving idiot in lane four. The sooner passing either side is officially adopted the better."
Undertaking is as I explained before - coming up behind a vehicle, moving to the inside and back out in front of the road hog. This is not legally permissable and is against the rules of the HC.
Driving along an inside lane, and coming across a road hog or queue of slower moving vehicles in any outer lane, and passing them on the inside is permitted - there is no rule or law against it. It is also basic common sense, as per Honest John's comment. It is also as I was trained to drive as an advanced driver and by the police.
Some people point to the "congested" part of section 268, which is one of those parts of the HC which is ambiguous and unhelpful. Most motorways we go on are congested, and every lane we see has a "queue" of traffic nearly all the time. "Queuing traffic" does not refer to any speed, nor does the definition of congestion. A motorway of cars with drivers progressing at 50, 60mph+ on the right hand lanes, all one behind the other, is queueing. If the inside lane is clear for some distance, you are not breaking the HC or the law by passing these slower moving vehicles on the inside, since as the HC says, you should keep up with traffic in your lane and it's downright dangerous and stupid to brake, move out past the road hog, then brake again and move out to the next lane past them, and all the way back again.
It's what many people do to "prove a point" to road hogs, but let me just say - don't waste your time, effort and petrol by doing this, and don't put yourself in danger by all these lane changes, just pass the idiot on the inside and maintain your driving speed. If you can catch the middle lane road hog on your dashcam and notify the police of their illegal driving - middle lane road hogging is an offence - do it!