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You must have a lot more faith than I do in the abilities of the absurdly-named UK Border Force to prevent people from entering the country unless they have proper papers. And much lower expectations that a well-funded terrorist organisation probably won't find it too hard to obtain false papers for its members anyway. Removing their nationality really will have no deterrent effect at all. It's a populist sop to the Daily Mail readers.
Er, no. Straw man alert.
Chopping people's heads off and engaging in acts of terrorism is against the law already. Removing people's nationality will not make it any more against the law. Preventing a state from removing their nationality will not make it any less against the law. Not removing someone's nationality does not mean that they are free to act with impunity and without trial, and so the fundamental premise of your rhetorical question is flawed to the point of non-existence.