Another factor now is Heathrow is more slot constrained. To get the flypast through they have to stop the Heathrow flow. For a nine element flypast (3min 30sec) like this one, Heathrow lose three arrival and three departure movements which they have to make up by stressing the system either side of the flypast. For the sixteen element flypast like 1990 Heathrow wouldn't now allow it. While you can make elements bigger (four ship/nine ship) we are never likely to see such a big one (in terms of separate elements) again over the Palace.