Thing is we don't have the death penalty for murder in the UK. I don't even think we even really have the law to enforce XPD (Expedient Demise) as Len Deighton wrote in the brilliant book (STBC here).
Why the death penalty applicable abroad then? That the state can kill its own citizen without recourse to a judge or a jury? We are not in a declared state of war. ISIS has made our own elected Govt. put us into a very dark place with this. ISIS have reduced us to this self-enforced low level with a few simple barbaric acts. Sorry Riley, we agree to disagree. Our elected MPs have to my knowledge never debated and decided upon this drastic course of state action. Forget John - it's the ground-stone basic principals of our British law and the role of the state which we seem to be dramatically abandoning very (far too) quickly.
Mach I think multiple issues are rapidly evolving. I don't think our UK mainland Police once a force, now a service (see that?) can presently even remotely cope with the present high threat if it becomes actual.
Forcibly very evident how well the rich and powerful in this country are guarding themselves at present. Cenotaph on RS; a ring of police/security around HM Queen, her subs and members of HM Govt. 6 hours later, I'm still in the pub, the Police have gone and we are truly on our own.
The media debate is very selfishly focused on guarding the capital and those most powerful who reside and work within it. What exactly about the other 50 odd million who reside here on the same islands? The runes don't read well.
One UK Police force in recent history couldn't even react correctly when 1 mad taxi driver went on the gun rampage in west Cumbria.
My local force have very few ARV, would take many minutes to form up and react to an Mumbai/Paris type incident.
I would seriously back well trained, experienced UK troops in an urban situation, because it's all we really have.
The authorities in the UK have had decades to come to some sensible arrangements, but seem to have achieved very little.
Concede it's a massive political loss of face to have to use military for domestic security and there we have the answer to the question.
* Apologies - been trying to post this back all day. But was blocked out from doing so by my companies own internet policy. I work alongside hundreds of French er workers, many with loved ones, friends etc in Paris... they bore up well - live news feeds must be torture.