A bigger question is why did this guy, who from all accounts was a pretty proficient operator, put himself in the position he did. He was not stupid and background chatter appears to put him in the camp of one of the "good guys". For some reason, he believed it would be worthwhile doing what was he was doing - what was that reason? Was this such regular occurrence he felt comfortable being where he was? Also, was he truly unaware of the buildings along the Thames? ATC were. The CAA were. Until we understand more about the person and the reasons why he was where he was (failing to adhere to Rule 5 is not a cause) we are set for a repeat.
A good report but desperately missing some personal, HF background and analysis.
PM