The Highway Code 163 says:

++ Only overtake on the left if the vehicle in front is signalling to turn
right, and there is room to do so
++ Stay in your lane if traffic is moving slowly in queues. If the queue on
your right is moving more slowly than you are, you may pass on the left

If you can legally pass on the right, why not do that instead of overtaking
on the left of vehicles? Am I missing something?