He didn't undertake, because that isn't possible on a single lane road. What he did was filter, perfectly legally.
I had a little google of the UK law, and not only are yoy wrong (see below) but stupid, as advising cyclists tofollow your cours woul get them hurt.
Anyway, overtaking onthe left/undertaking/filtering is not illegal in the UK, but it is not a god given right to do your own thing either.
The Highway Code says in section 139.
"only overtake on the left if the vehicle in front is signalling to turn right, and there is room to do so"
That is the relevant law in this case.
You also have to exercise due care and attention if you undertake and move into a junction when the traffic in front of you has stopped. If a car did it it would probably be seen as dangerous driving, I saw no cases involving cyclists.
Another case was a passenger who got out a car, and opened the door. An undertaking cyclist hit the door. The police judged the cyclist to be at fault.
I found a huge basis of complaints about cyclists undertaking when traffic is turning left, and it does not seem to be a slamdunk that the cyclist is in the right there either, the law seems to revolve around whether the turner in front is turning first and has used indicators or not, ie whether there was prior knowledge and the cyclist should have taken appropriate action.
Incidentally, I also found a few very useful cyclist websites advising cyclistson road lore, a lot more rational than the militants on this thread. There is hope.