Actually, the only airlines allowed to operate from the United States to London Heathrow are BA, AA, UA, and VS. AA aquired their landing rights through TWA in '91, as did UA from Pan Am.
Bermuda II also provides for only a certain number of cities which can have non-stop Heathrow service, of which the cities of Dallas/Ft. Worth were not included (I'll try to find a complete list). That is why DFW (and PHX, DEN, CLT, etc.) cannot have non-stop LHR flights.
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