Then it means he doesn't have that power. What kind of power is that which you can't exercise? If you can't exercise it, it means you don't have it.
Of course we do, the order would be ignored. (To whom would he even give the order?) English history is written in constitutional crises.
Well, the death penalty was abolished in the U.K. in the 1990s, so there's that. More significant: habeus corpus, which goes back to somewhere between the Magna Carta and Interregnum.
Even at its height, the Crown's power to unilaterally command executions was intentionally curtailed for centuries.