We haven't declared war, therefore we have no legal authority to kill anyone unless we are defending our land against an attack. If they are captured or if they surrender we have no legal authority to kill them.
Generally I'm in favour of bringing back the death sentence but spineless politicians combined with media control will never let it happen. Perhaps the fake News
media are frightened of being executed for treason.
I'm trying to remember if desertion is still a capital offence in the UK forces, I don't remember it being repealed. That would be double standards if its still in force.