Ghost-ryder
They don't need fighters. All they need is a light aeroplane or two, a couple of motivated individuals trained to a basic PPL standard, a target and BINGO!- An airborne suicide bomber.
Take a look at a Baltimore/Washington Sectional Chart or Terminal Chart and you will see that there are hundreds of small and private strips scattered around the area which could be home to a couple or couple of hundred light aircraft, some probably not used very regularly, and within a few minutes flying time of the SID/STAR tracks in and out of Dulles,or even the Final Approach tracks themselves for the various runways.
Personally,I think this smacks of a genuine received 'Carrier-Flight Number specific' threat either from a ground or external airborne source, due to the fact that neither HMG,BA or the US Authorities would allow the aircraft to depart for the return service to LHR.
...........Or...............
A new passenger vetting system with teething problems but why would pax. on this or the AF-LAX services trigger 'Flags' when many other carriers enter the US with more pax. with Arabic/Middle Eastern 'sounding' names?