You might have noticed that the only single-engined RAF aircraft that take part in Buckingham Palace flypasts are the BBMF fighters and the Red Arrows. They have a specific authorisation to do so, which is partly based on the pilots' status as specially-trained display crews. There are plans for emergency landing / ejection sites at various points along the route.
This is why the "EIIR" and "60" formations were only seen at the Windsor Castle flypast; the Hawks and Tucanos were flown by 'normal' instructor pilots and hence would not be authorised to take part in a city flypast.
When the Harrier was still flying, you wouldn't have seen that in a London flypast (not in the modern safety-conscious era anyway!) but Jaguars, Tornados and Typhoons have all taken part with their twin engines.