A couple of thoughts:
Some years back I was in this particular flypast; we knew we were in the 'right place' because we could feel the wake turbulence of the aircraft ahead of us.
Today - it looks as if the weather over London at the planned time of flypast was between BKN009 (EGLL) and OVC010 (EGLC). In my book that is a very good reason to cancel.
Safety Management, Risk Management, Threat Error Management or just plain Airmanship, call it what you will. To me, it takes bigger balls to cancel something like today than it takes to press-on.
