It's internationally illegal to intentionally make someone stateless. That is not disputed. You cannot make someone stateless on the assumption that they can apply for citizenship elsewhere. They haven't applied, they have no intention of applying and Bangladesh have consistently confirmed that, and said that her application would not be granted.
UK has no jurisdiction over Bangladesh's citizenship. It is fallacious to suggest it has. There was no mechanism for UK to remover her citizenship at the time, but they did it anyway, other than UK's interpretation of another country's laws.
The SIAC is a UK governmental organisation and their judgment on Bangladesh's laws is irrelevant and not impartial. It's like Sweden interpreting UK laws on migration, and making decisions that affect UK, based on their interpretation. It's nonsense to give credence to it.
Shamima wants to appeal the decision to strip her of British citizenship. She cannot do that from within the confines of the prison camp, and she cannot leave that camp without any citizenship. Bangladesh have confirmed that they will refuse her application, if she did apply. Just as UK have stripped her of British citizenship on National Security concerns, Bangladesh would be equally justified to refuse any application on National Security concerns.
The real issue, and the only issue is that Shamima already had British citizenship, she does not, never has, and would be denied if she applied for Bangladesh citizenship. Therefore she has been intentionally rendered stateless. An argument that she is entitled to citizenship of another country would be like arguing that asylum seekers must be accepted back in France because that was their most recent departure point. It's utter nonsense.
As far as the cost of justice is concerned, justice is priceless. But it would have been far cheaper to allow proper fair justice to take its course. The stripping of citizenship creates some disquiet about the potentially unfair treatment that Shamima has experienced at the hands of a media-led, governmental knee-jerk reaction.