To be honest I've never really liked London, it's a bit too fast and unwelcoming for me (from the times I've been there), nobody seems to have much time for each other. Barcelona's the place for me.
But I'm guessing you asked that question as a rhetorical one to get onto the story. It's another sad event and it's more than likely gang related, you get these in any major cities and considering the population of London is around 10 million, unfortunately, it is going to happen, as I've said in a post a while ago, I think the population impacts the required "nasty bah-stad" scale (i.e. it takes more extreme actions to be considered the "baddest" because there's more people to push it).
The only way I could envisage an end to these gang conflicts is to try and get them to mingle in some way, sounds so simple but I'm guessing it's easier for a member to attack a faceless member of another gang, once they know the person, maybe have things in common and like the person, it would be more difficult to attack them. But that's just my assumption, I think the only people who could ever give a valid answer are those who have first hand experience of what makes gangs tick.