So 'seatbelt' main reason for problem and then, quite apart from waiting for the Ambassador, the luggage being offloaded would have caused further delay (forgot about that).

From The Times
March 13, 2003

"Boy, 2, ordered off BA flight after tantrum

A BOY aged two and his grandparents were ordered to leave a British Airways flight after he could not be persuaded to put on a seatbelt.
Marcello Ferrand threw a tantrum and disappeared under his seat when crew on the flight from Milan to London tried to make him wear the seatbelt before take-off. The captain then decided that the party would have to be removed from the aircraft. It was delayed further while their luggage was unloaded......

....A British Airways spokeswoman said yesterday: “It is absolutely imperative for all passengers that they are wearing a seatbelt during take-off, landing and when the fasten seatbelt light is turned on.”