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This is the nub of the issue.Helmets may help anyone who suffers a head injury by whatever cause. Drivers head hitting the windscreen when bumbed from behaind by another driver, someone in a bub brawl who dies, someone falling down the stairs due to a trip hazard in a building, and if a third person is involved in some else head injury would their responsibility in law be diminished it the victim was not wearing PPE in any other instance? Doubt it.
It is farcical that only in cases of bike riders incidents that this is even considered a factor
It is possible that the police believe that a helmet could have prevented death so a charge of "causing death by dangerous driving" might not be appropriate. If they think that they should let the judge and jury examine the evidence and make a decision. It does not explain why there was no prosecution for careless driving.
http://lcc.org.uk/articles/dead-cyclists-family-let-down-at-every-stage-by-met-police