I'm thinking perhaps the pilot would have been visual with the surface for the turn to Battersea, saw that he was higher than the top of the building so no problem, (didn't see the crane going up the other side), held the angle of bank in and through that soft cloud that was above the building and didn't expect a jib arm hidden in the cloud.
My opinion it was simply bad luck, should not have gone through the cloud, but bad luck all the same. (How many of us fly a 180 through a cloud and expect nothing in there?)
His experience should have won the day. Dam crane was the problem.