I've know the PM use a BA 767 as well but it certainly wasn't especially kitted out for him.
This debate always stalls on the cost. It has been shown time and time again that for essential travel either schedule or charter is more cost effective than ownership (although some sort of assured contract or PFI might bring the cost down). I had the honour (sic) of going on a No 10 chartered aircraft - in fact we were told we had to use it. The cost charged per seat to our budget was more than the BA flex business fare! Still getting in and out of the destination airfield was much slicker than travelling commercial.