It isn't Cameron's aircraft it is ours.
He has an incredibly difficult job to do and how he does has a huge effect on all of our lives.
The plane is just a tool to help him do this job better.
It is great pity that because of stupid, ignorant class warfare in Britain we have underprovided for many years with air transport for senior politicians and civil servants. Virtually every other country does it better.
With the current ISIS threat in the world and our misguided involvement in their conflict Cameron is now a prime terrorist target. This aircraft helps to mitigate the risk.