In answer to your question IslipWN, the U.S.-U.K. Bilateral Air Treaty provides for only two U.S.-based airlines and two U.K.-based airlines to operate routes between the U.S. and London Heathrow Airport.
United purchased Pan Am's LHR
route authority in 1991. American, as a competitive response to the UA
/Pan Am agreement, purchased TWA's LHR
routes shortly thereafter (also in 1991).
Currently, only British Airways, Virgin Atlantic, American, and United are permitted to operate routes from LHR
to the U.S. Over the past several years, talks between the U.S. and U.K. governments to expand access at LHR
, have proved unsuccessful. Hence, today, only AA
are allowed to operate from the U.S. to LHR
United's B747-400. "She's a a cruel lover."