I've been involved in one teenage violence incident, and I do think you have options other than just fleeing.
It was very fast, no time to find out whether I'd have managed the level of intervention you did, Junglist1 but ... what horrified me, and I mean horrified, was that people stepped over the boy who was down on the ground. People were pushing past us, tutting. The child/adolescent was bleeding.
People freeze, and they don't want to get involved. These were children, but big children. I suspect that, because I have a boy of my own, I could see they were boy children, and that's how I reacted.
I can see that people want desperately to think this is "someone else's problem", and use all sorts of distancing devices in order to protect themselves, and reassure themselves but it can be really unhelpful.
And look, part of the whole "Oooh Londoners" thing is a distancing move. It slides into "Well, you have choices - if you don't want this sort of thing to happen to your kids, don't live there", which then slides into "Well, you bring it on yourselves by living there".
London is a big city: 8 million? 10 million people? It is that big because it has a lot of jobs there that people can't do anywhere else. We can't all move to Cornwall. And why should we?
There is a problem with child on child violence. Just turning our heads away and singing "la, la, la" isn't going to get us very far.