Finally note that British Airways is not a US carrier and not even
required to
follow 14 CFR 121.

Untrue.
121.1
This part prescribes rules governing--
....
(f) Each person who is an applicant for an Air Carrier Certificate or
an Operating Certificate under part 119 of this chapter, when
conducting proving tests.

119.1(a) This part applies to each person operating or intending to
operate civil aircraft -
(1) As an air carrier or commercial operator, or both, in air
commerce;

So if they want to operate as an air carrier in the U.S. then for the
portion of their flight in U.S. territory they are indeed subject to 14
CFR 121.