The first six Tristars were the definitive BEA order (a prototype came over to the UK with large BEA decals on it prior to the full BA merger), later ones went to BA mediumhaul routes (eg Gulf) for which they were more appropriate.
The -500s must have seemed a good idea at the time for long thin routes but turned out to be too expensive; the probably did not have much resale value and the RAF requirement was too good to miss.
The Tristar later came on its own transferred to the BA charter arm, Caledonian, who seemed to make money in a really competitive market for many years with it.
I took a -500 in the early 1980s on London-Seattle-Vancouver, when BA were just starting to build these two destinations. As always on the Tristar, it seemed to have a stylish cabin interior compared to the rival products.
No I am not just writing that to toady to 411A
