The 1970s Delta 747s at Heathrow were on an "interchange" flight with Pan Am. Delta had no licence to London then, but the flight routed London-Washington-Atlanta, with a full Pan Am crew, and on their AOC, for the transatlantic sector, and likewise crewed by Delta onward to Atlanta. Both Pan Am and Delta marketed it as a through flight. The daily rotation just fitted one aircraft, which was provided by the two carriers in proportion to their route mileage, so about 75% of the time it was a Pan Am one and 25% it was a Delta one, generally in blocks of months of one, then the other. It had been like this before 747 days as well, done with DC8s. Delta didn't have 707s so Pan Am had to hang on to some of their older DC8s when they otherwise withdrew them. Both airlines needed to have the aircraft type fully on their AOC.
Eastern Airlines were an early Boeing customer for four 747-125
Eastern did a notable on-off with the 747. They ordered four early on, but before they were built changed their mind, decided to wait for the Tristars, and sold the build slots to TWA. Then the Tristars were delayed with the Lockheed/Rolls-Royce issues, so leased a couple of 747s from Pan Am early on in partial livery (both were blue of course). About 10 years later they thought they were going to get the Miami-London licence, so ordered a couple of 747s secondhand from Qantas which were actually painted up in full Eastern colours in Sydney but never delivered as the route permission was unexpectedly not given to them. None of these ever got anywhere near London.