Henry
You have seen the red arrows low level over central London, yes?
Sometimes in close formation with Concorde etc?
Red Arrows and everyone else do
flypasts over central London, not displays, and certainly not aerobatics. Very, very much lower risk.
AFAIK they do those flypasts at about 1300 feet, which hardly qualifies as low level?
Somewhere in the risk assessment of those Red Arrows flypasts must be consideration of flying single-engine over London (not normally allowed). Perhaps the reliability of the engine in straight-and-level flight is a factor, similar to the risk assessment of ETOPS.