i assume that someone with real knowledge will come explain this in a minute, but my understanding for now is that there are now 4 possibilities: - they keep negotiating and find a new brexit deal before the deadline and pass it in parliament (lol good luck) - they convince the eu to give them an extension on leaving (maybe) - they just up and leave the eu without any kind of deal, which fucks over british trade and immigration for, like, a while (nobody seems to rly want this, but it's what happens if nothing else changes, so it's a real possibility) - new referendum and hopefully a remain vote this time (seems unlikely? but maybe)
The important thing to keep in mind is that none of this stuff is really sussed out. The EU referendum was effectively "do you want to break this?" and 51.89% of voters said "yes." Not clear at the time - and not clear now - is that once broken, there isn't a lot of law or agreement describing what you do with the pieces, where you sweep the dust, what you replace it with, etc. The EU has little incentive to help out the UK on this one. Much of the UK has little reason to sign on - reefing back, saying "hold another referendum" or otherwise saying "hell to the no" about anything that comes through brings things closer and closer to calamity so there's a fair amount of brinksmanship. It's bad enough that the PM might end up instituting war powers to mandate a bunch of law by fiat and the UK may/may not fight a war over Malta/.