Some years ago, this Govt had to decide about a replacment for the Royal Yacht (Brittannia) then at the end of it's econimical working life. There were economical and poltical and emotional reasons both for and against. They decided not to replace it. In my view this was a mistake.
If they tried to get a PM's flight, there would be mayhem in the streets. We have small machines in the Queens Flight for the Queen's Prime Minister to use. Renting bigger machines, when needed, is the only sensible way to go. As mentioned, the press pay an exhorbitant price for their seats and that covers most of the cost.
There will be detailed plans for war (all-out, as opposed to the current wars) and that will doubtless include RAF machines with secure comms etc.
I have not yet taken part in a protest march but a request for a PMs aircraft would have me shouting in the streets!