BA was the launch customer for the -500. They chose it over the DC10-30 with RR engines though its payload range capacity was less than the DC-10-30. Political reasons almost certainly formed part of the rationale. Additionally BA had orders and options for -1s far in excess of shorthaul needs so converting these orders to the -500 made some sense.
The six initial -500s received by BA failed to meet their performance guarantees, allegedly by nearly 10%. In consequence they were returned to Lockheed for retrofit of the extended wingtips and active ailerons and the lower sfc -524D which became standard production features. Even so, they could not operate LHR-LAX without a payload penalty.
BA then decided they would prefer the higher capacity but shorter range of the -200 series. Eight of these were delivered.
The sale of the six -500s and the simulator to MoD was controversial at the time because they were sold at what was said by some to be a firesale price. Allegedly BA had an urgent need to shore up its balance sheet pre-privatisation and MoD was a willing customer.
Subsequently BA leased in two late production -500s from Air Lanka for use on the South American routes acquired from BCAL. These operated to a 6000 kg higher MTOW than the original BA -500s. This made LHR-GIG-GRU possible, though often it was a a close run thing.