In Socimer International Bank Ltd v Standard Bank London Ltd  EWCA Civ 116, per Rix LJ:
It is plain from these authorities that a decision-maker’s discretion will be limited, as a matter of necessary implication, by concepts of honesty, good faith, and genuineness, and the need for the absence of arbitrariness, capriciousness, perversity and irrationality. The concern is that the discretion should not be abused. Reasonableness and unreasonableness are also concepts deployed in this context, but only in a sense analogous to Wednesbury unreasonableness, not in the sense in which that expression is used when speaking of the duty to take reasonable care, or when otherwise deploying entirely objective criteria: as for instance when there might be an implication of a term requiring the fixing of a reasonable price, or a reasonable time. In the latter class of case, the concept of reasonableness is intended to be entirely mutual and thus guided by objective criteria. ... Laws LJ in the course of argument put the matter accurately, if I may respectfully agree, when he said that pursuant to the Wednesbury rationality test, the decision remains that of the decision-maker, whereas on entirely objective criteria of reasonableness the decision-maker becomes the court itself.