Interesting notes above. I think the first US scheduled carrier at Gatwick was Braniff in 1971. Passing through this period with a wide range of US operators as documented, we are back today with none. Sure there are the UK-based operations to some US leisure destinations, but the previous tradition has gone and these flights take few American travellers. I can't recall who were the last US carrier to depart. The old Gatwick Express train, with the British traditional doors where you had to slide down the window and reach for the outside handle on arrival, was always a particular challenge for them, and I, and doubtless others here as well, over time released quite a number of stuck US travellers from the train
There was however a considerable US presence before 1971, as it was the major UK point for affinity charter operators, or "Supplementals" as the US described them, principally in the summertime. Names such as World, Capitol, Transamerica, Universal, American Flyers, and even some new names towards the end in the 1980s like Arista, with the stretched DC-8 becoming their aircraft of choice. These carriers had as their all-year work flights for the US military when they had a major presence in Europe, with Frankfurt as a principal node but also other points as well, even Mildenhall, and they, doubtless with some scheduling dexterity, managed to link this work, a lot of which was required one way, with the commercial operations on the return leg. You might find various morning positioning flights from Gatwick to Frankfurt or vice-versa, and other combinations, tying it all together. Actually not always that dexterous, there were plenty of accounts of long-arranged summer affinity charters being pushed by one or two days either way to suit later US military requirements, or departures 8 hours late which, few were told, was actually prearranged, to suit crew overnighting times.