BOAC was gone before the first TriStar was delivered! They were merged with BA in March 1974. BA didn't recieved their first L-1011 until November 1974.
You are quite correct but in fact the airline still operated internally as Overseas Division [ BOAC] and European Division [ BEA] for many years and so when the Tristar [ ordered by BEA for European routes ] started to operate outside of the old BEA area of the world, the aircarft became an Overseas division aircraft and so should be crewed as an Overseas aircraft with a F/E.
I think it was in 1982 that the unions and management agreed that all 3 man crew aircraft within British Airways would be crewed with 2 pilots and a F/E rather than with 3 pilots. Not much of an achievement for the F/E as British Airways never from then bought a fleet of aircraft with a 3 man crew.