I'm just quoting the court, pitchfork-bearers. From a more detailed summary issued by the court:
[They start by saying the case shouldn't even have been heard because she didn't bring her appeal under the right statute - but brushing over that...]
Secondly, the Court of Appeal erred in its approach to the appeal against the dismissal of Ms Begum’s application for judicial review of the Secretary of State’s refusal of leave to enter the UK [134]. It made its own assessment of the requirements of national security, and preferred it to that of the Secretary of State, despite the absence of any relevant evidence before it, or any relevant findings of fact by the court below [108-109, 134]. In particular, there was no evidence before the Court as to whether the national security concerns about Ms Begum could be addressed and managed by her being arrested and charged upon her arrival in the UK, or by her being made the subject of a Terrorist Prevention and Investigation Measure [109]. The Court of Appeal’s approach did not give the Secretary of State’s assessment the respect which it should have received, given that it is the Secretary of State who has been charged by Parliament with responsibility for making such assessments, and who is democratically accountable to Parliament for the discharge of that responsibility [134]. Thirdly, the Court of Appeal mistakenly believed that, when an individual’s right to have a fair hearing of an appeal came into conflict with the requirements of national security, her right to a fair hearing must prevail [110, 135]. But the right to a fair hearing does not trump all other considerations, such as the safety of the public. If a vital public interest makes it impossible for a case to be fairly heard, then the courts cannot ordinarily hear it [91-94, 135]. The appropriate response to the problem in the present case is for the deprivation appeal to be stayed until Ms Begum is in a position to play an effective part in it without the safety of the public being compromised [135]. That is not a perfect solution, as it is not known how long it may be before that is possible. But there is no perfect solution to a dilemma of the present kind. In those circumstances, Ms Begum’s application for judicial review of the LTE decision was properly dismissed by the Administrative Court [111], as should be her cross-appeal in respect of SIAC’s preliminary decision in the deprivation appeal [97].
Thirdly, the Court of Appeal mistakenly believed that, when an individual’s right to have a fair hearing of an appeal came into conflict with the requirements of national security, her right to a fair hearing must prevail [110, 135]. But the right to a fair hearing does not trump all other considerations, such as the safety of the public. If a vital public interest makes it impossible for a case to be fairly heard, then the courts cannot ordinarily hear it [91-94, 135]. The appropriate response to the problem in the present case is for the deprivation appeal to be stayed until Ms Begum is in a position to play an effective part in it without the safety of the public being compromised [135]. That is not a perfect solution, as it is not known how long it may be before that is possible. But there is no perfect solution to a dilemma of the present kind. In those circumstances, Ms Begum’s application for judicial review of the LTE decision was properly dismissed by the Administrative Court [111], as should be her cross-appeal in respect of SIAC’s preliminary decision in the deprivation appeal [97].
Fourthly, the Court of Appeal mistakenly treated the Secretary of State’s extraterritorial human rights policy as if it were a rule of law which he must obey [136], as opposed to something intended to guide the exercise of his statutory discretion [122]. On a deprivation appeal, SIAC is not entitled to re-exercise the Secretary of State’s discretion for itself. Rather, unless there is an issue as to whether the Secretary of State has acted in breach of his obligations under has the Human Rights Act, SIAC is confined to reviewing the Secretary of State’s decision by applying essentially the same principles that apply in administrative law [66-71, 118-119]. In this case, having considered detailed assessments by his officials and by the Security Service, the Secretary of State was not satisfied that depriving Ms Begum of British citizenship would expose her to a real risk of mistreatment within the meaning of his policy. SIAC decided that that conclusion was not an unreasonable one. There was no defect in SIAC’s reasoning in that regard [130]. Ms Begum’s application for judicial review of SIAC’s preliminary decision in the deprivation appeal is therefore dismissed [131].