Just to add what has already been said, ignoring charters Pan Am had 2 stints at Gatwick, 2 July 1980 they started Gatwick to Houston, I believe this lasted until 1983 and was flown by L1011s and 747's, as has been mentioned Houston could not be served from Heathrow due to Bermuda II and by merging with National Pan Am gained quite a presence at Houston.
They returned in May 1991, having sold their Heathrow rights to United, ending service to Heathrow on 3 April 1991 however there was a gap in service as I presume they hand't been able to set up operations at Gatwick in time so United actually flew Heathrow Miami for a month. United did not buy the Miami and Detroit routes so Pan Am started these from Gatwick from 18 May 1991, Detroit was an A310, Miami a 747. Delta bought all of Pan Ams transatlantic routes and took over from 1 November 1991 ending Pan Ams presence in London.
Some of the dates might be slightly out but that is a rough idea.