I don't agree that those are necessarily the only options -- you could also try to foil the attacks as they come.

Regardless, my qualms are basically two. Firstly, the decision to use lethal force on a British citizen (whereas in Britian we don't have the death penalty). Here, I can see some argument regarding practicality, as you have made.

Second is a qualm with the process in making that decision. The lack of a police force in Syria really doesn't affect the British judicial process (after all, there is precedent for in absentia trials). I am simply not very confident in the process regarding terrorism charges. In large part, this is because of the way that the government has used terrorism over the last 14 years: it has consistently been one of the chief rhetorical justifications of a movement towards the erosion of people's rights. It is something like the case of the boy who cried wolf. I have heard so much rhetorical exagerration of the terrorist threat to freedom (often in order, it seems, to impose upon that freedom) that it becomes difficult not to be skeptical of any case of the government justifying any of its actions via protecting our country from terrorist plots.