This is the question cyclists ask most, and the answer is more reassuring than most people expect. There is no legal requirement to wear a helmet when cycling in the United Kingdom, so not wearing one is not an offence and does not prevent you from claiming.
If the other side raises it, the argument is limited in two important ways. It applies only to a head injury, not to injuries elsewhere. And they have to show that a helmet would actually have prevented or reduced that specific head injury, which is a higher bar than insurers sometimes imply. A neck injury, a broken wrist or a leg fracture is unaffected by whether you wore a helmet.
The same reasoning applies to high visibility clothing and lights. If it is argued that you could not be seen, the question is whether you could reasonably have been seen given the conditions, the lighting and the driver's position, not simply whether you were wearing something bright.