They have been led to believe they can have some kind of fantasy deal that would be even better for them (no payments to Europe so the NHS can get more, control

They have been led to believe they can have some kind of fantasy deal that would be even better for them (no payments to Europe so the NHS can get more, control over immigration, but all the pros of the EU unchanged), but forget completely that that would not be "win-win" from the viewpoint of the EU and thus won't happen. They're not revising their agreement, they're kicking it out entirely (work that took decades) and want to re-start on their own terms. People can make choices and there's nothing "wrong" about that, but some choices are smarter and less damaging for everyone involved than others.