That realsitcally flew a constant heading and hight untill intercepted. I guess the terrorists have been taught to fly straight and level.
I don't think anybody in the RAF is really expecting a terrorist attack. What they are surely expecting is a bunch of PPL holders infringing on the prohibited zone either because of poor navigation or because they didn't bother to read the bloody NOTAMS. Those will indeed be flying straight and level at least until interception, afterwards they will be flying rather more erratically as their hands won't be as steady any more. The wet feeling in their pants will also serve as a major distraction, somewhat detracting from their otherwise impeccable flying skills.
Cheers,
DG800