Is there a phrasing you think would be neutral that still references that data point?

I'm struggling to figure out a neutral phrasing involving this data point. My reading of your response indicates that you see a temporal indicator ("now") or a delta indicator ("still" / "no longer") as both introducing bias. "One in five people think Brexit was the wrong decision" without anything clarifying significantly misrepresents the data.

It seems your issue is actually with the data point being in the headline at all. Maybe the headline should be a dry thing like "A Brexit survey happened, read inside for details". I won't entirely disagree. But I think this is less about word choice than you think.