She voluntarily gave up her UK citizenship. She didn't believe herself to be British anymore. She thought the Islamic State was something permanent as many did.
She may have been born here, but neither you, I or even the British Government can force someone to be British if they declare themselves not to be. When the Supreme Court says that her rights do not trump the rights and safety of the citizens of the UK, you know you've backed the wrong horse.
Remember she thought nothing of the rights of others in her pursuit of her "perfect society", and to deny those, whose regime she supported, that were victims of said regime, the justice they deserve, because of... i'm still waiting for a legitimate reason on that.
She's 'British'? Not anymore. The Supreme Court has ruled in favour of that. By 'law' that should be upheld. Unless you and others know better than the Supreme Court. Trust me, had the SC ruled in favour, i'd have said nothing, but those arguing against the SC's ruling today, I have to ask why.