Not strictly true: it is legal to overtake on the left hand side on a one-way road.
I was told by a UK policeman some years back, during a lecture on road safety, that there is not an explicit ban on overtaking on the left on a motorway. He then pointed out that a motorway is really 2 one-way roads side by side. I think if you were to be charged for overtaking on the left, it would be for some vague offense such as "dangerous driving", "driving without due care and attention", etc..
I think it is explicitly legal to overtake on the left if there are
2 or more streams of traffic and the left hand stream is moving faster than the right-hand stream.