She doesn’t have it currently. All children born of Bangladeshi parents anywhere have the option to apply for their citizenship, but she has never done so. She is solely British at the moment, well, stateless really.
The government spotted a loophole to prolong the situation. Bangladesh have already said that her application for citizenship will be rejected.
I guess she will have to formally apply and get rejected before the government will have to concede that they’ve made a publicity stunt and repatriate her, like countless other people that went to Syria to fight for ISIS.