Just a random thought I wonder whether anyone has thought of installing a transponder (or similar device) in cranes over a certain height. I'm not sure if this would be useful in the TCAS environment.
TCAS 1, the type fitted to helicopters, is a very useful aid to safety. However, it still requires the pilot to visually aquire the transponder, or rather whatever it is fastened to, and avoid it. In this case it would be of no practical use. This accident involved a known object but the pilot appears not to have visually aquired it.
Speaking as someone regularly flying rotary in the London airspace, often to and from Battersea, using this very route, and often in the same type of aircraft as the unfortunate deceased pilot, I'd say high intensity strobe or LED lighting on the crane, so that it could have been recognised as such, would have been of far more practical use. The usual "obstruction light" is a single red light. These lights do not stand out.
A classic example is the full-width arch over Wembley Stadium. This is an open structure, very similar to a crane. You can fly very close to it but not see the red light (in fact I think there are two) because they are not bright enough against the background. However, occasionally, the whole structure of the arch is lit by a series of bright white lights. It can then be (and is) used as a navigational feature for miles around.