If the general flow of traffic on the left is faster I see no issue with passing on the left. The big problem is with those who weave in and out, often with no indication and with not enough notice to react even if they do indicate. Typical example I saw on the M6 near Corley, dawdling git in an Alfa 146 in the middle lane, a Golf GTi came up behind him like a sh!thouse rat and started to undertake just as the Alfa driver moved over, having seen said Golf closing in. Very nearly a nasty accident, and both drivers would in my view have been equally to blame had it resulted in a collision.
There needs to be as much emphasis on lane discipline as there is on speeding, drink driving and using telephones. Poor lane discipline leads directly to road rage, with undoubted escalation to more serious incidents.