Section 40 of the British Nationality Act
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You cannot deprive a person of citizenship if you make them stateless. Added to this the Convention on the Reduction of Statelessness means you cannot deprive a person of their nationality if that would make the person stateless. There seems to be a heavy reliance on her Bangladeshi citizenship. If she doesn't hold another citizenship but would be eligible for one, then the Home Secretary can only deprive her of her British citizenship if she was naturalised as British, because the effect of the deprivation would make her stateless. But it's hard to see, based on the above how she could be a naturalised British Citizen as she left when she was 15.
Bangladeshi law, and I don't claim to have any knowledge whatsoever about this, seems to follow biologically, so she may automatically have Bangladeshi citizenship despite not identifying as such.
The Pham case probably serves as a good guide for the Home Office Decision
Full case here: