I don't even go along with that. The chances of legislation coming about has receded to the far margins of probability, but, just to hypothesise, if legislation were passed the chances of it surviving for any length of time would be slim. The immediate fallout - a reduction in the number of commuting cyclists, the loss of bike shops and the hit taken to sustainable transport (which, for all their shortcomings, all parties have signed up to) would be bearable - but I seriously doubt whether the police would enforce it - they've got enough to do.
You'll doubtless consider me an optimist, but I do seriously think that a helmet law would bring about a result that very, very few people, other than a few rabid cyclist haters, want