Quite so, BA hired in daily an Air New Zealand DC-10 (-30 ) from about 1975 to 1981 to run Los Angeles to London, which they felt was too much range for their 747-100s. In fact it was more than one overall, as the eastbound flight left LAX several hours before the westbound got in. Air New Zealand had an extended layover at LAX in the day, so it suited them too. BA provided full crewing for this; there's a regular poster over on PPrune who was one of the BA pilots then. Initial training for some months in New Zealand, they went for their regular simulator checks to KLM in Amsterdam.

When BA started getting their more capable Rolls-Royce engined 747-200s, LAX was the first route they put them on, plus they started a daily SFO as well, but the agreement still had a while to run, so the DC-10s still did LAX twice a week to change over, and in fact they increased the number of hours contracted, the aircraft now also did, at varying times, Boston, Miami and Montreal. Air New Zealand had their fateful sightseeing DC-10 crash in Antarctica in late 1979 which had a huge impact in a small country (and in fact continues to do so), and their business fell right away for a considerable time so they were glad of the extra utilisation. When there were two ANZ DC-10s at Heathrow they were invariably parked next to each other. Air New Zealand disposed of their DC-10s quite soon after the Antarctic accident, which had tainted their name, replacing them with Rolls-Royce 747-200s, and BA returned the favour by training their initial crews.

I was a bit of a regular on the BA LAX DC-10s in the late 1970s.