Per Lord Hope in Chargot Limited (t/a Contract Services) and others, R v  UKHL 73 at para 27:
27. … The first point to be made is that when the legislation (The Health & Safety at Work etc Act 1974) refers to risks it is not contemplating risks that are trivial or fanciful. It is not its purpose to impose burdens on employers that are wholly unreasonable. Its aim is to spell out the basic duty of the employer to create a safe working environment. This is intended to bring about practical benefits, bearing in mind that this is an all-embracing responsibility extending to all workpeople and all working circumstances … . The framework which the statute creates is intended to be a constructive one, not excessively burdensome. In R v Porter (James)  EWCA Crim 1271 the Court of Appeal set aside the conviction of the headmaster of a school where one of his pupils lost his footing on a step which gave access from one playground to another while he was unsupervised, with tragic consequences. It held that there was no evidence that the conduct of the school had exposed the child to a real risk. The situation was not such as to give rise to a risk of the type that section 3 identifies. That was an exceptional case, but it makes an important point. The law does not aim to create an environment that is entirely risk free. It concerns itself with risks that are material. That, in effect, is what the word "risk" which the statute uses means. It is directed at situations where there is a material risk to health and safety, which any reasonable person would appreciate and take steps to guard against.
Per Lady Justice Sharp in C-T Aviation Solutions Ltd v Regina (Health and Safety Executive)  EWCA Crim 1620 at para 27:
27. The source of the risk to safety must be the undertaking conducted by the employer, and the risk must be a material one, in that it must be a real, as opposed to a trivial, fanciful or hypothetical one.