I've written before about being in Edinburgh in the 1970s when the new runway was planned and built (in surprisingly short time compared to how these drag on for years nowadays). It was strongly argued that the old short runway (13/31 ?) was unable to handle Tridents. Once construction was agreed however they started to filter in. The last Vanguard service, fractionally into BA times, was June 1974, and the London service was then all Tridents. I don't think they were Trident 3s. The Shuttle to London, Trident first sections and backups, started in summer 1976, a year before the new "Trident capable" runway even opened.