The burden of proving causative contributory negligence rests on the Defendant. Stanton v Collinson and Smith v Finch both emphasise the importance of medical evidence that without the fault the injuries would have been a good deal less severe. In the case of helmets the type of injury is important; most neurosurgeons will accept that a helmet may avoid lacerations or a skull fracture or an extradural / subdural haematoma but few will argue that they can protect against coup or contrecoup type injuries or diffuse axonal injury.
Some neurologists take the view that a helmet may increase the risk of a rotational type injury to the brain because the diameter of the head is effectively increased. Most medical experts will wish to defer to the expertise of a cycling expert on the efficacy of helmets and consequences of their promotion.