A former crewmate of mine told a story about taking Margaret Thatcher in a VC10 to Washington to meet the US and French Presidents.
They were late leaving Heathrow due to road traffic and a man came to the front asking if it would be possible to arrive before the French Presidents aircraft.
The crossing was made at about M .91 and mission accomplished. (not much fuel remaining)
He also told me that there were significant position errors on the VC10 at high speed so that when indicating M .91 you were actually doing M .88.