The only BA TriStars that tech stopped at Bangor were the British Airtours aircraft in 392 seat configuration en route to Orlando and LAX. KT had a large charter operation to both destinations and Bangor was a scheduled stop for fuel and crew change. It was one of those situations - allegedly - where the economics of the tech stop were equal to or better than the nonstop, though of course it was aircraft performance that dictated the need for the stop.
In mainline BA operations to the USA the L1011-200s served Eastern Seaboard destinations as far as Washington. The -500s went as far as Seattle and New Orleans. I believe BA's original specification for the -500 was for LAX nonstop but because of the performance shortfall BA never attempted this. In any case the route needed the greater capacity of the DC10 and 747.
The -500s that flew the Rio route were not the original BA aircraft which by then had been sold to the MoD but two late production aircraft leased in from Air Lanka. These had a higher MTOW (c.+6000kgs) which was often necessary for the route. Even so it could sometimes be a tight operation. Southbound, early morning fog at Rio could be an issue, resulting in the occasional diversion. Northbound, take off from GIG could variously be limited by MTOW, RTOW (WAT limit I think) or fuel tank capacity. The Sao Paulo shuttle was planned to return to GIG with fuel to Max Landing Weight. Supposedly the rationale for this was that the GRU fuel was colder and further cooled by the brief cruise at FL370 hence a greater fuel weight could be loaded at GIG for the LHR sector if tank capacity was an issue.