Yes the Prime Minister is First Lord Of The Treasury as it say's on the big black door. I understand the PM is not head of state, the Sovereign is. I didn't say the PM was.
The PM though is the leader for all intents and purposes of the democratically elected government of the UK. I am not going to get into the detail of constitutional responsibilities. The PM is basically representing the people of the UK politically and is the person primarily responsible for government policy.
The Sovereign and the PM need good and capable transport that is up to the job for the tasks required, it is as simple as that.
It also has nothing to do with the individual man/ women or their policies but with the position of Sovereign and PM.