I think it is generally accepted that already when Brexit was up for vote it was known to be a universally bad idea - from a rational point of view. Almost every trustable source of information was clear that, for example, the economic outcome it would lead to would be ... suboptimal.
So, if you were rational it was pretty easy choice that Brexit would be the wrong way to go.
Unfortunately, people are not rational (and, even worse, unwilling to admit not having been rational at the time) and many could be "captured" by all kinds of vague BS that politicians came up with -"take back control"- that sounded as if Brexit would be a good idea.
(Actually the UK has less control now of global matters, since they can't shape EUN decisions anymore, the UK has rather given up control - to France and Germany which now dominate EU decision making.)