Recently in Lancaster, a driver suffered a seizure at the wheel and his car mounted the pavement, hitting a couple of pedestrians... one of which suffered a head injury and was taken to A&E. The very same courts would find it laughable if the driver's insurance company claimed contributory negligence against the injured pedestrian.
Though your point is completely valid, there are some cyclists even, who think it it stupid not to wear a helmet, but silly to suggest pedestrians should. Unfortunately judges are typically not well informed, so if an insurer suggests cyclist should "obviously" have been wearing a helmet, as "encouraged" by the highway code, judge might conceivably be swayed but this . The insurer's brief could easily be more slick than the claimant, especially if the later is dead. The statistical argument that they don't make a differnce would only sway scientifically minded people - judges are typically not scientifically minded - hey doctors are typically (but not invariably) inumerate in thes things, so there's even less hope from judges.
This isn't to say, "wear a helmet", but more to stress that we need to be constantly vigilant against creeping compulsion,
