The grounds on which a High Court judge can overturn the decision of a government decision-maker include, among others:
that the decision-maker was mistaken about the facts or about the law
that the decision-maker took into account irrelevant factors, or ignored some relevant factors
that the decision was made for an improper purpose
that the decision-maker didn’t follow the rules of natural justice – for example, they were biased against you, or they didn’t give you a chance to put your side of the story, or they changed their policy without publishing the new policy.