Yeah, considering what's been happening in London over the last 18 months, it's probably a bad example as a comparison. Still, I never feel unsafe walking the streets of London and have many times in Buenos Aires (ignoring the obvious that all cities have very bad places where you shouldn't go). It would be interesting to see how crime rates in these two cities compare over recent years. I am as guilty of this as anyone, but I believe it is largely unfair to compare Buenos Aires to a city like London or indeed Argentina to the UK/Europe (although I think economy is fair game).
That said, in London, I would class myself as unlucky if I was a victim of crime. Here I think I am lucky that I have not been one. That's the difference for me. I live in Versalles, which is hardly crime central, but still hear of 5-10 incidents in this neighbourhood every week. Just cause they are not on the news does not mean they don't happen. It's the same in Devoto, Villa Real, Villa Crespo, Palermo, and Flores where I have friends who report a similar number of incidents.