The UK is a good example of how much damage you can do yourself (nationally) by just disregarding facts and reality for a few years. Our own biggest enemy is our massive arrogance.
I presume you're referring to Brexit, but this seems to be mostly a strong US dollar and energy crisis story. Europe is in a similar situation so I'm not sure what difference staying/leaving would have made.
In this specific case it's not really a "strong dollar/energy crisis" story – the proximate cause is a massive overnight shift in UK government fiscal policy to "let's cut taxes and not spending, and it will all work out" that's obviously spooked everyone.