possel,
You are right about the early Trident debacle, but the VC10 was a commercial disaster from the start. It was built for use on "Empire routes" to hot and high airfields with short runways, something that disappeared before it was in service as everyone lengthened runways for Comets, B 707's and DC-8's, and the "Empire" routes were not the cash cows that an airline needed to make money, and they went the way of the Empire anyway. As a result the VC10 was hugely uneconomical compared to the Boeing 707 and DC-8 on routes that really made the money for the airlines. Yes they were popular with passengers, and of course BOAC were going to mount a campaign to get folk to fly them after the Government insisted they operate them, but they were not as economical as the competition both external to BOAC and internal with the Boeing 707. The ultimate judge was the market, and Boeing sold 865 707's to airlines, Douglas sold 556 DC-8's while Vickers managed to sell a paltry 40 to airlines, most of which were not wanted by the airline that bought them.
And as for building for one airline and to their requirements, why was it always the British airlines and airliner manufacturers that got it so wrong? Why on earth didn't Vickers build for THE one market that mattered, the North Atlantic?