I'm still surprised about any ex BA 747/RRs being leased out in the early 1980s. BA had a considerable need for these aircraft for longer haul routes where their original 747s did not have the capability (California/JNB/Far East nonstops), but they were severely restricted by government ownership on capital expenditure to about two new frames a year, when they wanted more.
The BA/AE 757 arrangement that came along in 1983-5 allowed AE to charter out their new 757 capacity in winter, while BA aircraft went the other way in summer. The upside for BA was the much more efficient 757 during the winter; in the summer and shoulders when the 757s went away they reinstated their much less fuel efficient Tridents, the last of which were withdrawn at the end of 1985 which coincided with the end of the arrangement. There must have been some sort of "by the hour" charging as the AE 757s seemed to dominate on BA's shortest 757 sector, the Shuttle from Heathrow to Manchester.