AA operated the 75L from JFK to STN (London Stansted) for a couple of years around 2006-2007. It's difficult to see it as other than an attempt to drive Maxjet (startup cheap all-business airline) out of business, though at the time the Transatlantic market was booming so more seats made some economic sense. I used this several times to get to BOS - the tossup being whether it was more hassle to go to nearby STN and deal with JFK immigration and a flight to BOS or travel to the further LHR but take a direct LHR-BOS flight. The novelty of the 75L usually won out for me.
I recall the flights being rather quiet, load factors were never very high. Business class was particularly deserted, which I'm sure affected revenues badly. Even without very high status in those days, I still got upgraded to the NGBC seat more than once. An AA lounge was under construction in 2007 or early 2008 when the route was, fairly abruptly, pulled. I suspect having driven Maxjet under there was less need for it, but the oncoming recession was already beginning to bite so it wasn't surprising that AA closed the route even without Maxjet as competition.
By 2010 the idea of any transatlantic flights from STN was laughable, traffic was down 25% from 2007 as everyone felt poorer. STN traffic figures did not exceed 2007 levels until 2016, driven by the rise and rise of Ryanair, several leisure-focused airlines, and a rotating parade of startup airlines from Turkey.
We still have no transatlantic flights from STN. The only long haul flight is Emirates to Dubai, although I don't expect that to survive the coronavirus.