"Reasonably practicable" is the term used by the H&S fraternity to allow a job to be done as safely as possible. Looking at the RAF approach, it seems that the best way to avoid the problem is not to allow single engine aircraft in the area. That in fact prevents the job being done in the first place. Now if you dont need to fly over London, there is no problem, but the chances of a single engine aircraft crashing elsewhere are the same no matter where you are, but it still dosen't seem to happen. If the air marshals choose not to let aircraft with one engine fly over built up areas, thats fine and dandy, but it is not a logical decision based on risk, it is a reaction based on likely consequence.
If the moon fell out of orbit and hit the earth it would kill all life on the planet (consequence) However is it ever going to happen? (risk). Well it might I suppose.