Per Lord Sumption in Hayes v Willoughby  UKSC 17 at para.14:
14. Reasonableness is an external, objective standard applied to the outcome of a person’s thoughts or intentions. The question is whether a notional hypothetically reasonable person in his position would have engaged in the relevant conduct for the purpose of preventing or detecting crime. A test of rationality, by comparison, applies a minimum objective standard to the relevant person’s mental processes. It imports a requirement of good faith, a requirement that there should be some logical connection between the evidence and the ostensible reasons for the decision, and (which will usually amount to the same thing) an absence of arbitrariness, of capriciousness or of reasoning so outrageous in its defiance of logic as to be perverse. For the avoidance of doubt, I should make it clear that, since we are concerned with the alleged harasser’s state of mind, I am not talking about the broader categories of Wednesbury unreasonableness, a legal construct referring to a decision lying beyond the furthest reaches of objective reasonableness.