<<Do you know if there is a blanket ban on single-engined flight over the city? Presumably "specials" like Battle of Britain Flight get dispensation anyway, but banner towing??!!>>
One of my currently active colleagues will hopefully reply but I do not know the current situation. When I was operational there was no such restriction providing the pilot could obey the Rules of the Air. No flights by light aircraft were permitted below 1500 ft over Central London but plenty operated at 1500ft.