She didn't have Bangladeshi nationality, she has a tenuous link to Bangladeshi nationality through a parent or grandparent, I don't know which.
Much the same as the English lads who end up playing for the Republic of Ireland football team.
If I'm not mistaken, the Bangladeshi government have refused to recognise any entitlement she might have to Bangladeshi nationality.
So that makes her as British as you, me or anyone else with a British passport.
The British government can't just wash her hands of her.
A case in some international court would be interesting.