Just because a route is "possible", with ATC clearance, it doesn't make it legal or sensible. In fact, I would go as far to say that flying a single engine aicraft such as this, at 1,200 feet over London is pretty reckless. This is not like passing over a small town, there are NO forced landing areas in Greater London, apart from farmers fields on the very outskirts (which this particular aircraft would not have been able to glide to)
Yes, we can all "move on", but when a Piper Arrow tries a forced landing in the local park and ends up wiping out your family/ending up embedded in your house, when it could easily have taken a longer route, you may think different.
(See Airmanship/Threat and Error Management)