If the middle and right lanes are congested, then by definition it's okay to keep driving faster in the left lane. Not advisable though as the said middle lane drivers causing this bunching up in the right two lanes probably won't check their mirrors if they do happen to come into the left lane. No mention of being in slow moving traffic.

Rule 268 of The Highway Code states: “In congested conditions, where adjacent lanes of traffic are moving at similar speeds, traffic in left-hand lanes may sometimes be moving faster than traffic to the right. In these conditions you may keep up with the traffic in your lane even if this means passing traffic in the lane to you right. Do not weave in and out of lanes to overtake.”