SB is a British subject, and the British Government is in breach of international law in making her stateless.
True, but that piece of international law wouldn't have the slightest bit of democratic legitimacy. You're talking about elites signing a treaty when 99% of the public at the time could have thought that principle to be nonsense. I suspect a majority of the public today would support being able to remove citizenship of people where they join an organisation at war with us.
To strip them of citizenship, is just to formalize their own choice. They have already chosen to renounce their allegiance and loyalty to the UK. If you have no loyalty to a nation, and indeed are committed to its destruction, you can't really be a "citizen".
That said, as I say, I have a little bit of sympathy for her as she made the mistake when still a child. Well all do stupid stuff as kids; although mostly, it doesn't go as far as joining with terrorism, mass murder, taking sex slaves, and trying to build a fundamentalist religious fascist state.
Maybe it is worth thinking what she experienced in this country that so alienated her that she thought supporting Muslims was a priority.
Well she experienced freedoms and e.g. legal rules that you can't discriminate. Very different things to the "Islamic State" she went out to support. Maybe some people were nasty to her; that happens to plenty of people for various reasons. That doesn't normally result in joining a murder cult. And for all I know, her life in the UK was normal and fine.