It is still not our responsibility to police other pilots, this is not a big brother state.
When I was granted my PPL I was under the understanding that I was responsible for my own actions and the fact that I had demonstrated both academically and physically my understanding of the rules and skills that I was capable of making my own flight safety decisions.
It is too easy for us to sit in judgement of others from the safety of the internet. We were not at the controls and therefore I ask how we can question someone elses decision making process.
Having never flown over that section of London I have no idea if there is glide clear space or not. Looking at the half mil chart not a lot of green is seen but looking at an OS map of the area it seems to me that there are plenty of green spots to "drop" into. If I was in an engine failure situtation I would not be concerned about a graceful landing but rather the saving of life and limb, both mine and others.
As a point as well it is also remarkably difficult even for a pilot to judge the height of an aircraft from the ground, a fact ably displayed by more than 100 members of our club at a recent fun day. Not 1 person was able to corectly judge the height of a Cessna 152 being flown over the airfield at 2000ft within a 1000ft!
It is hard to see unles we have followed the same route from the air ourselves we can make comment?