First flight of my life, aged 9, on a VC-10 to JNB, also on the Super. As I have read ... the VC10 was engineered for many of the Hot & High fields of the Empire. These had short runways (natch). So the engines needed to get the new machine off the ground in that space had to be powerful.
The Americans took a practical alternative: Pay the foreign countries to upgrade their airport to the new international standard to cope with the jet age and provide longer runways. Once that was done, who are they going to buy their new jets from...?
By the way, in apportioning blame for fouling up the UK air industry, don't leave UK GOV out of it, they had their fingers in ALL the pies (airports and carriers) and made as big a mess of it as they later did to the nationalised UK car industry.
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"I tell you, we are here on Earth to fart around, and don't let anybody tell you any different."
Kurt Vonnegut, Jr.