same here
passing on the inside and undertaking aren't the same though
passing on the inside is fine
Technically it is fine ONLY if the traffic in the right hand lane is in a long queue and is travelling slower than you in the left hand lane, then you can pass legally.
Rule 268 of the Highway Code states – do not overtake on the left or move to a lane on your left to overtake.
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.
Overtaking on the left is lawful if done in accordance with the above guidance. However, whilst there is no specific offence of overtaking on the left, if a driver undertook another vehicle, other than in the circumstances described above e.g. in the case of someone hogging the center lane on a motorway, that is otherwise clear in all lanes, they may be prosecuted for careless driving, as it is perfectly possible for them to move to the right most hand lane pass the slower vehicle in the center lane and then return to the left most lane.
Yes its a PITA but it is also the only completely legal way.