60% chance: similar shit show continues until March 2019. Huffing on both sides, probably some walking out, little progress. Economic outlook worsens considerably in that time. Inflation up, pound down, companies moving jobs out, public spending low as everything worsens. Public sentiment looses enthusiasm for hard Brexit, or perhaps Brexit at all as reality bites.
in a couple of panicked all nighters as the deadline approaches, a deal is thrashed out. Yes, it would be better for us, for them, if we thrashed one out sooner. But the EU never gets serious until the deadline, and does all its deals on the eve of them (see also: Greece). Deal involves staying in Single Market on basically the same terms for the next few years, but without voting rights. Maybe something on future relationship. Likely the beginning of a rolling never never where politicians can claim Brexit has been achieved without actually leaving the single market / EU changes enough with a range of 'membership' options so that the whole concept of Brexit becomes moot.
E.g. meantime core group of EU countries press on with further integration UK would never have agreed to.
Eventually some sort of trade deal agreed which us and other non EU core countries settle into. Economic decline bottoms out sometime during this period and recovery begins. New normal for next 10 years plus.
So rough, but survivable.
20% chance: either side comes to senses and deal gets thrashed out according to original timescale. Could be better or worse than the above. The one benefit of the first scenario is the hangover will have kicked in and killed off all notions of hard Brexit.
20% chance: no deal. Big negative economic impacts for UK. Beginning of serious decline. Public services as we know them (NHS etc) fall apart, as they are simply unaffordable in our new economic reality. Fall out of the G7 sometime in the 2020s. Eventually admit our mistake and rejoin the Single Market sometime towards 2030.
On the plus side: in any of these, the death / split of the Tory party. Possibly Labour too. Realliagnment of british politics: for me, Brexit is best seen (from a UK perspective) as the end of the Thatcher paradigm. As, in turn, the arrival of Thatcher was the end of the post war paradigm. Not clear what the new British settlement will be yet, but likewise it wasn't really clear what Thatcherism was about until the mid 1980s.
Like it or not, Brexit forces a European reckoning too. The victory of Macron allowed entrenched Euro elites to think it was a blip. But they are wrong, and the sentiments expressed by Brexit are growing elsewhere across the EU, and not going away (see most recently the German election result). TO deal with this, the EU will HAVE To change to survive, and that will be a good thing.
But potentially a shit decade meantime. Which is non trival.