These questions keep coming back, for whatever reason.

In the case you present there, the ex-PM has no more power to command any servant of the Crown (Ministers, advisers, civil servants, police, army). No one will obey him. As I've said before, he can hide in the broom cupboard at N° 10 if he wants, but some discreet political policemen will come to take him away.

Unless (as I've said before) the tanks are in the streets on his behalf. Don't forget that the Queen is also C-in-C of the armed forces. Mutiny against the Queen? Not going to happen.

Things are going to slide, slide in all directions
Won't be nothing, nothing you can measure anymore
L. Cohen