BOAC were the first airline operator, but not the only one. BOAC didn't want to operate British built aircraft & went out of their way to sideline those that they were made to take. The VC10 was another they didn't want, as was the Brittania. They were only happy when they got the Boeings that management had wanted all along.
The RAF used Comets as passenger aircraft & transports until the mid 70's, not just the Nimrod.
Nothing like an early set of problems to commercially doom an airliner as an airliner - Lockheed's Electra was one that was, like the Comet, subsequently developed into a successful maritime reconaissance aircraft after early crashes in airline service.