Boltar ) gurgled happily, sounding much like they
were saying :

shrug
How often do bus engines require full power? Not very.

Agreed but when they do need it they really need it. The steep hills
around mill hill, highgate , hampstead and southgate spring to mind. A
hybrid bus with an exhausted battery going up one of those hills won't
be very popular with other motorists.

Nor would a diesel bus with an empty fuel tank...

I really don't think it's going to be an issue.