What "Spit the Dog" might not appreciate is the difficulty of conjuring up a C17, Tristar or VC10.
Just adding a few extra tons of gas to the return trip to the UK for a timed flypast, just might mean that not everyone can get a seat home. How happy (or legal) do you think that the soldiers on board might feel about making their loved ones at Brize wait for another hour whilst they participate in an airshow?
Whilst your idea sounds simple, in reality every little change has far reaching consequences, legalities and difficulties. Such is the reality of the modern military force.