This is the issue, she’s Bangladesh’s problem, not the UK’s. If Bangladesh had revoked her citizenship first she would be able to come back but they didn’t.It's also an interesting (at least to me) but entirely academic point (because it will never be tested) that Bangladeshi law says that citizenship must be renewed at 21 if a citizen holds dual nationality. However as she had been deprived of her British citizenship by the time she reached 21 she did not hold dual citizenship and therefore is arguably still in law, a Bangladeshi citizen.