Apparently, it was very foggy. Pilot didn't see the crane (or saw it last-second without enough time to evade), impacted, couldn't regain control, slammed into the pavement. This seems like a non-issue. Accidents happen.
He couldn't go to high because that is where all the air traffic is and he couldn't go to low because of the crane. But pilots say that it is very hard to fly over London.