As Dallas says, it changed about 5 years ago, but up until then, any Airmiles you earned on MoD business were supposed to be "donated" to your approriate Transport Command who re-used them for "official" business. All absolute nonsense of course, but some lower grade civvie bods I worked alongside in the secret procurement facility near Bristol used to religiously turn them in. I destroyed the evidence of my wrongdoing by cashing them in for a flight for Mrs TI, worked fine for me. A sense of righteousness must be a terrible burden to bear.