SKipness
You're absolutely right. If the head of state, or the PM is strutting the world stage it shouldn't be on a commercial airline. Imagine arriving at, say the G8, when Putin, Obama, Hollande, The Japanese PM (can't remember his name) and everyone else arrives in "State" transport, and Cameron turns up in a commercial airliner that simply doesn't stand out from the crowd - plenty of dim journos (and there are plenty of dim journos!) would probably think he'd turned up on a scheduled service.
Hardly the impression a world player (if that's what the the UK really is) wantsto give.