Yeah I know why people could think that but I think its a minority. I think the majority of British public see cycling helmets and just see it as the norm. I think they're more likely to go "Look at that idiot not wearing a helmet" If they were to see someone with a helmet I don't think most people would be thinking "it must be dangerous if they have a helmet". So I think the fear of needing a helmet is already there with a lot of people from a young age are taught you must wear a helmet or if you fall off you will land on your head and die. Speaking from a generation of this being engrained into my head, 90's kid n all.
So to clarify I think seeing someone with a helmet on doesn't change someone's mind that much (Bright protective gear probably does more) and its the fact from an early age we're taught cycling requires a helmet as general PPE as you're likely to hit your head is what will put people off.