Sorry if this seems a bit "anorackish" but as an outsider the reason single engine aircraft can't fly in certain areas (Northolt/over London/anywhere you like) is to do with risk. The people who make these rules dont seem to understand risk much, and it looks like the rules were put in place years ago when it could be argued the risk was much higher than today.
We can keep a few civil servants busy for a month or so collating how many flights are undertaken by single engine military aircraft per year. (Bound to be lots) Then how many of these have engine failures (not many I think) then work out the percentage risk of this happening (very small) Now even if an engine failure happens, how many result in crashes (i.e if you can dead stick it in somewhere, or do you jump out) this again reduces the figures. Even in the worst case, when an aircraft is abandoned and crashes, how often will this result in injury/loss of life to those below? When you add this all up I rather think you get a risk factor which is so small it is insignificant. So whats all the fuss about?? When you boil it all down, you will find that some senior bod somewhere thinks his career is on the line should it ever happen that somebody gets an aircraft through their roof. In fact the chances of this actually happening are so small you can ignore them. All part of risk management.- Ah yes but what if it does happen ???