Per Lord Sumption in Hayes v Willoughby  UKSC 17 at para.14:
14. Rationality is a familiar concept in public law. It has also in recent years played an increasingly significant role in the law relating to contractual discretions, where the law’s object is also to limit the decision-maker to some relevant contractual purpose: see Ludgate Insurance Co Ltd v Citibank NA  Lloyds Rep IR 221, para 35 and Socimer International Bank Ltd v Standard Bank Ltd  Bus LR 1304, para 66. Rationality is not the same as reasonableness.