Airbrake,
On the face of it and if Brian Abraham has accurately posted the FAR then it would appear that it is legal under FARs to complete the flight on three.
It is also the case that the airline's AOC is issued, not by the FAA but by the British CAA.
Clearly, a sovereign nation must have some say in what takes place in its national airspace but it appears odd to me that: a) BA should be fined over a matter of pilot in command judgement which did not even appear to contravene FARs, and b) the matter was not dealt with at ministerial level to decide how future similar occurences were to handled.