The original BEA order was for TriStar-1s for the European network.
BA then added TriStar-200s for long haul routes ( I flew LHR-Kuwait-Dubai-Dehli and Bangkok-Dubai-LHR in 200s) as 707/VC-10 replacements.
I believe the -500s were bought for South American routes.
Fantastic view from the jumpseat on a TriStar. The seat was raised up so that you could see over the Captain's shoulder and the window beside you came right down to your waist. Felt like you were almost floating in the sky!