The issue really is due process and I guess it's something we should all be bothered about that is what distinguishes us from them, however I doubt they considered themselves British citizens and they had effectively performed an act of treachery by going out to fight with ISIS
More interesting to me and more worrying for ISIS is how we got the intelligence on them, most likely they have been infiltrated at quite a high level (and low) considering the Government had a meeting before authorising the attack they must have known their whereabouts and movements for quite some time
People say they (ISIS that is) prefer martyrdom, well they say they do but you don't see many of the high level members volunteering for it, it's mostly brainwashed dupes that get done whilst the high level live it up in relative safety, I don't think they have been sleeping well for some time now