By the time a regime reaches "existential threat" level it's too late. You're already far gone, any and all political opposition has been driven underground and the regime has loyalists in place in all institutions and positions of power. There is no way to organize any resistance because attempts to do so are crushed immediately, the press is controlled and you're in the minority anyway because the majority are propagandized and kept in fear of instability or supposed external threats to their way of life.

The EU is frankly far too close to that dystopian end already. It constantly justifies its own power via reference to external enemies of various kinds e.g. capitalist America, the European press barely challenges it and the institutions are controlled by EU loyalists who are willing to rip up the rules rather than let the EU lose control. Even in the UK that was always more distant than other nations, they went through multiple severe constitutional crises triggered by Remainers fighting to overturn the referendum. It wasn't at all clear that the UK could actually leave despite the referendum result not being in doubt.

IMO the very fact that you think the UK left for "questionable reasons" is already part of the propaganda doing its job. The people who get out early are the only ones who actually can get out. By the time the case is unambiguous (to you), you're the one being labelled as questionable.