TWA operated from LGW - JFK with daily with a Tristar in summer 1985 and possibly a couple of years beforehand - I would have to check old timetables.
Interestingly TWA operated positioning flights between LGW & LHR for the 747s operating the STL services, presumably for maintenance purposes.
I think that I saw three Pan Am charters at LGW on a Saturday in the 80s. The fourth PA flight was the scheduled L15 from IAH.
I think that People Express started transtlantic ops with a single 747 and they chartered a Pan Am 747 one day to cover for their plane going tech one day when I was on the viewing deck (interestingly handled by Gatwick Handling who handled PA rather than British airways who generally handled PE.) I don't know if this was a one off or happened more often.
As an aside, I remember seeing a United Airlines 747 on a charter from Chicago in the 80s, long before they regularly served the UK.