Can back that up with a quote from Roadcraft The Police Driver's Handbook.
Lane discipline on motorways (ch 12, p217):
There are no slow or fast lanes. Overtake only to the right, except when traffic is moving in queues and the queue on your right is moving more slowly than you are. Do not overtake by using a lane to your left.
Therefore, if you are travelling at 70 mph in lane 1 and a car is travelling in lane 2 at a slower speed you may pass without overtaking, but do so with caution in case numpty pulls across suddenly into lane 1....you may be in his blind spot.
If travelling in lane 2 behind numpty, overtake using lane 3 only.
Them be the police rules!