That did not set a precedent for helmetless contributory negligence. The judges remarks in relation to the lack of a helmet were obiter dicta in that case and are not binding on any subsequent case.
I think you are misunderstanding the meaning of precedent.
GC
It is a case in the High Court and until the Appeal or Supreme Court reverse it it is the precedent. And I have spent far too many hours with leading QCs in the High and Appeal Court not to know what the meaning of precedent is. As for obiter dicta, it was addressing two of the principle issues of the case and set out the judge's conclusions on those principal issues i.e rationes decidendi not obiter dicta and therefore setting the precedent.
The principal issues
<....>
v) If the Defendant's negligence was primarily responsible for the collision, was there any contributory negligence on the Claimant's part?
vi) Did the Claimant take reasonable care for his own safety by not wearing a cycle helmet, and if so would his head injuries have been prevented or reduced in severity had he been wearing an approved helmet so that it would be just and equitable in all the circumstances to reduce his damages.
and from the Case Digest:
Summary: Despite there being no legal compulsion for cyclists to wear helmets, a cyclist not wearing a helmet was, like a car-user not wearing a seatbelt, exposing himself to a greater risk of injury and ran the risk of contributing to his own injuries in the event of an accident caused by another road user.
And to answer the other posts on this at the same time, there are two separate steps involved. Smith v Finch determined the principle that not wearing a helmet can be contributory negligence by the cyclist with a consequent reduction in damages in line with the principles set out by the Appeal Court in Froom & Ors v Butcher [1975]. But then the facts of each particular case would determine whether or not there was a contribution. In all of the cases so far, the Court has concluded either that a helmet would not have made any difference in the circumstances or that the Defence had not demonstrated to the Courts satisfaction that it would have made a difference.
Thus from the Case Digest for Smith v Finch:
On the balance of probabilities, S had hit the ground at a speed greater than 12 mph so the wearing of a helmet would have made no difference to the injuries sustained. Moreover, the scalloped shape of most modern helmets would probably not have prevented S's injuries, given the location of the impact on the back of his head. Even if the impact speed had been low enough for a helmet to have afforded protection, F had adduced no medical evidence to support his case that S's injuries would have been reduced or prevented by his wearing a helmet. Accordingly, F had failed to discharge the burden of proving contributory negligence.
I suspect that if the Court were faced with a simple fall within a helmet's certified design parameters, or an on the ball Defence and not on the ball Claimant the outcome may not be so favourable.