I'll marry you bae, then you can become a US citizen overnight!
Depends on what you mean by on their own. If you're talking about total isolationism, then yeah it'd pretty be difficult. I will say that UK was the fifth largest economy (Still is maybe? I think the pound has already bounced back to almost normal levels) in the world, maybe it was a bad decision to leave, maybe not - either way I think they'll get their shit sorted and be ok at the end of the day.