Exactly this , I think we have to let her back unfortunately because she only had British Citizenship, we cannot refuse her entry as it’s illegal under international law. So make her face prosecution.Nothing to do with Brexit mate and nothing to do with our laws. I think it's international laws that mean we have to take her back in. Could be wrong of course, but it will be something to do with ensuring that no person ever becomes country-less, so to speak. Which makes sense.
Just bring her back and stick her in prison for joining ISIS in my opinion. Gets her off our streets and it's not worth the hassle battling with international law.