Please point to the law / section of the Highway Code that outlaws
'undertaking'.

Highway Code paragraph 139
"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
give motorcyclists, cyclists and horse riders at least as much room as
you would a car when overtaking(see Rules 188, 189 and 191"

So unless in slow moving queues or undertaking a vehicle indicating a
right turn the highway code says don't do it. These rules are advisory
however in if an accident was caused by a driver ignoring them he
could be charged with
driving without due care and attention
Iain