The Highway Code rule on this is rule 268, which 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 your right. Do not weave in and out of lanes to overtake.”
This applies to cyclists in a cycle lane next to the kerb, who may well be able to travel faster than traffic in the all-users lane to the right.
In other circumstances, cyclists should pass on the right. In my experience, doing so is sometimes hampered by motorists who respond to noticing me coming up behind them by moving a little to the right. At first I thought that they were doing this deliberately to make passing them difficult, then realised they were probably making misguided attempts to helpfully share the road, and were assuming I was going to pass on their left, when I had no intention of doing anything so silly.