[dragonwriter]

> Except many judicial reviews aren't about the legality of the decision, merely the way the decision has been made

But isn't that an essential feature of the constitutional structure of the UK: given that the UK doesn't have a separation of powers system in which various entities within government have limited powers, any decision by government that is within the sovereign power of the government is substantively legally, the only question of its legality is whether or not it is a valid decision of the government, made by the means legally prescribed by the government for the kind of decision it is -- but this can look at like review of the legality of the substance of the decision, because the legally prescribed mechanism for a decision may vary based on the substance of the decision.

(In a sense, this is true of any sovereign government, even, e.g., the US separation of powers system, wherein the "way the decision has been made", in order to be legal, for certain decisions involves a Constitutional Amendment.)