The purpose of a system is what it does

So what's Brexit going to do?

Looks like it's going to do two things inescapably; create great food insecurity, and gut The City of London as a world financial centre.

If I thought there was a cohesive English Landowners Party, I could think this was what they wanted; they can get a very cheap labour force out of people who would prefer not to starve, and they can get a permanent shift in the domestic balance of power in their favour.  It's an utter disaster on an international scale, but they might not care.  (Expecting ANY long term agricultural productivity in England as the climate shifts could be considered rather daft, but it'd be a highly plausible daft.)

So the current "what are they thinking?" resolves to Brexit starting to look like something that a bunch of white supremacists used Russian backing and dubious data expertise to enact, and the Landowner's Party grabbed it.  Whatever the cynical participants wanted to extort has gone by the wayside; this is now about bringing back forelock-tugging agrarianism on pain of death.  (Well, by means of death.)  And no one political can figure out how to say so at a profit, and no one political is going to do anything that doesn't profit them.  The opposition can't figure out how to think about the whole thing, because the idea of no more foreigners is just too attractive for words to too much of the electorate, who stop there.  There is no way to champion free movement and win an election.  (It's looking like you might not be able to say the Scots aren't foreign and win an election.)

Globally, converting the UK's not-quite-four-percent of the global economy into despairing noises won't be good.   By itself, not as large as 2008.  In combination with a collapse of the trade regime, though?  That could be pretty thorough.  Lose the trade regime and the ability to impose sanctions won't survive, which is something Russia would want.