But you don't need a helmet on a motorized trike.
So something like an ICE Fat trike with a big motor and a fairing to deflect the bugs would be road legal, once you comply with all the other paperwork, without a helmet.
I agree. But a trike has built-in stability from its three wheels, which a
bicycle does not have. But I couldn't ride a trike here. I ride in lanes only one trike wide and soon a farmer on a tractor, literally overlooking me, would run over me. On my bike my whole head clears the top of a Range over, and I present a profile that no one can doubt will damage even a sturdy tractor.
Note that where I live a cycling helmet is not compulsory, though it is advised by the police if you ask them. In fact, where we don't have a specific law, if you say, "Oh, this is okay in England," or "Man, this is okay in Germany. See the CE and TUV marks?" that's a convincing argument. Not that I've ever been stopped on my bike, not even once in nearly thirty years of cycling here, except by policemen who wanted admire my bike.
But the people I cycle with think nothing of embarrassing a clergyman for asking for salt at dinner; you can imagine how they would react to anyone who turns out for a ride without a helmet. It would be worse than turning up without your trousers, which would be overlooked as mere absentminded eccentricity. Not that I normally succumb to that sort of peer pressure, but I'm very fairskinned, so I burn even on mild, overcast days, and am always under some kind of a hat, on the bike or off it. For me a helmet is a skinsaver, mandated or not.