British Airways did not have the DC10 in its fleet during the 1970s, it only operated the type after taking over British Caledonian in 1988. BA was actually anti DC10 and would have preferred the A300, but was forced by the government to buy the Tristar in order to help Rolls Royce who supplied the engines.
The L1011 was phased out from the BA fleet four years before the DC10 as the DC10 had the longer range and was the only alternative to the B747 until the T7 arrived.