Well there is a security risk involved in chartering a plane, it is very slight however and some of the concerns that affect a charter plane affect a dedicated one too. Also there is a comms issue as I have a nagging doubt about the PM communicating on HF (shortwave) via Shanwick, and I don't know how secure the usual BA at seat airphones is. Yes he could use Iridium etc. but again I don't know how secure they are, however Brenda IIRC has one for use on the royal train, and the US gov makes extensive use of them, so I would suspect that there is mod for them.

Will we have fighters on alert at the nearest airbase and an AWACS/Nimrod shadowing the plane when it flies as the US do?