Nasty cycle accident on Kentish Town Rd last night

Not convinced. I think there are cases when undertaking is fine. What's the official line (e.g. Highway Code). Not that I think the Highway Code has got everything right, just wanted to know what the law is.

• 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