Combination of sources inc "Speedbird" - the BOAC history - and the 1999 British Airways Airlife book.
BA entered into the ANZ DC10 exchange as neither the 747-100s or 707-436s had range to operate non-stop to LAX, but BA was facing competition from Pan Am and TWA who were flying 707-300s non-stop.
Operations were on LHR-LAX from May 1975.
For Summer 1978, the arrival of the BA 747-236s provided aircraft which could fly LAX and this was the first route on which the -236Bs were flown. There was a further year on the interchange agreement still to run, and the ANZ DC10s were flown on five Miami and three Montreal flights per week plus two LAX to feed aircraft back into the ANZ network. The agreement ended in April 1979.
Can't vouch for any of that from personal experience but that's what the history books say!