Interesting question. A motorway carriageway is ipso facto a one-way system. You are allowed to 'undertake' on a one-way system. Indeed, the Highway Code says that 'drivers on a one-way system should be aware of vehicles overtaking them on the left'.
This didn't stop police in Dundee recently charging a motorist for overtaking on the left on a dual carriageway. The driver claimed he had thought the other car was going to turn right at the next roundabout. (This would have made it OK). But the police said 'Rubbish', and charged him anyway. Me, I'd have thrown the Highway Code at them and the magistrate and told them to read it.
On the other hand, some time ago I heard a Chief Constable talking on the radio about motorway 'undertaking'. He said that whilst there was no law against it, if it resulted in an accident, 'the driver's action would be taken into consideration'.