What you believe has nothing to do with reality though. In every jurisdiction where helmet wearing has been made mandatory and enforced cycling has dropped significantly and particularly in the under 16s where decreases of up to 90% have been seen.
A study of UK towns showed that cycling declined in those where helmets were promoted but grew in those where they weren't.
Several cities are now repealing mandatory helmet laws because their Boris Bike equivalents are incompatible with mandatory helmets. Those that haven't, most noticeably in Australia and New Zealand, have bike hire schemes that are largely unused compared to the great successes in London, Paris, Dublin, Barcelona.......
Have a look at this Australian view of Dublin compared with Melbourne and the impact of mandatory helmets.
I'm not sure it matters whether what I believe has anything to do with reality in this case. In truth, what I believe has quite a lot to do with reality, but in this case I may be mistaken. My comment began:
"I don't believe for a moment that cycle helmets will become compulsory, but if they do then very few people will stop cycling."
I continue not to believe that cycle helmets will become compulsory. It is likely my belief will prove well-founded.
In the unlikely event that I'm wrong, I believe there will be very little impact on the numbers who cycle in the UK.
Indeed, as energy prices rise and cycling becomes more accepted I wouldn't be surprised if compulsory lycra failed to put people off.
Studies are huge fun on the right day and comparisons between studies are great when it's raining, but nobody I know would stop cycling because of a helmet law. Most of my friends are not keen cyclists. They just ride bikes because it's fun or makes sense. They would continue to do so.
I need not go beyond the conditional tense because I'm confident that no such Act will be passed in the UK.
You commented: "A study of UK towns showed that cycling declined in those where helmets were promoted but grew in those where they weren't."
I don't live under a blanket and I do occasionally watch the News, but I'd never realised that there were two distinct types of town; those where helmets are promoted and those where they aren't. That's the sort of statistic that gives statistics a bad name. In fact 73% of statisticians have grave misgivings about such data.
Promoted how? By whom? To what end?
Not promoted how? Not promoted by whom? Not promoted to what end?
How can two towns in the same study either promote or not promote helmet use with equal vigour.
Over what period of time? In which season?
It isn't going to happen - and if it did the impact would be minimal.
There was a huge and passionate debate in the mid-80s (when I was a bike courier) about proposals to introduce mandatory leg guards on all new motorcycles. The debates, petitions and heated arguments were endless and hilarious.... but it was never going to happen and never did.