Political bias aside for a brief moment I do believe that a bigger aircraft should be available for the PM (of any side).
A 737 with a bed in it doesn't leave much room for anything. I wonder what the 737 crew do for crew rest on it. A aircraft properly fitted with crew rest AND properly crewed would increase the safety and capability of the VIP Sqn.
Just maybe a extra A330 might fit the bill here.
It could also be used at short notice for emergencies and not affect the KC role of the other 5.