In my opinion, there are three ways this could turn out:
Leave could prevail. In the short term, there would be much uncertainty; the pound would take a hit and there'd be a near-term economic downturn. Perhaps Prime Minister Boris Johnson, his Brexit gamble having served its purpose, would fudge some kind of reconciliation with the EU, or perhaps the UK would still be out. All those Leavers who were looking forward to an end of the effects of globalisation and a return to jobs for life and old-fashioned community values would be in for what Milton Friedman might have called a short sharp shock; the new Britain, afloat on the high seas of international finance, would have to compete on some basis, and being a gateway to the European Union (which, for all its problems, is a huge economy) would no longer be that basis, so it'd come down to low wages, lax regulations and/or tax-haven-style opacity. A permanently low pound (and the absence of any automatic rights for British citizens to look for work abroad) would ensure the first point; the other two could come as political necessity, as governments, needing to attract business, cut everything from human rights to environmental regulations. So, post-Brexit Britain would look not so much as a cozy worker's utopia in vintage bunting as a dirty sweatshop and equally dirty tax haven, whoring out both its historic reputation and its captive population. (As one might expect, Russia's oligarchs expect a bonanza after Britain votes to leave the EU; the depressed pound will let them snap up more prime London property, and the receding threat of transparency rules will make London a very comfortable environment for the wealthy and corrupt; for the rest of us, not so much.)
(That is only considering what would happen in the UK itself. In Europe proper, Britain leaving would embolden its own anti-EU fringe elements, the Marine Le Pens and Alternativen für Deutschland and the don't-call-us-Nazis sticking their noses into the tents of government all over the Nordic countries. Once, say, France or Germany left, the EU would effectively be finished as a significant entity. So border posts go up, cooperation gives way to competition and mistrust, and soon armies are being built up, just in case the neighbours try something. Meanwhile, on the Eastern fringe, Russia jockeys for control over the Baltic states. Poland decides, for perfectly understandable reasons, that it needs a nuclear arsenal, then Germany (hemmed in between nuclear France and nuclear Poland) decides it needs one too. Finland starts preparing for another bitter winter war. And around Europe, nationalist parties are on the rise, and skinheads are attacking foreigners, liberals, gays, and anybody outside a narrow view of their country's national identity. The old post-WW2 world of Interrail and EasyJet, of Erasmus scholarships and weekenders at Berghain and complaints about drunken English stag parties, will seem like a long-lost golden age, and the future will look like the millennia-old killing field. In this world, even if you did manage to get an EU passport before the door slammed shut, Europe won't be a welcoming place to go.)
Remain could, narrowly, win. 51-49, 53-47, or similar. Everyone breathes a sigh of relief for a moment, given that the UK is not crashing out of the EU. The Brexit faction, being reasonable people, realise that the people have spoken and accept their vanquishment graciously, dissolving and going away. Boris Johnson and Nigel Farage audition as presenters for the next reboot of Top Gear. Britain's bout of anti-European mania is over, as the nation looks to embrace a progressive, inclusive vision beyond its borders.
Who are we kidding? Almost half the population will have voted to leave the EU; the right-wing press are still around and still despise the EU and the progressive impositions it makes. There would be calls of fraud, demands for a recount, perhaps even allegations that MI5/Mossad/the Masons tampered with ballots. (It's likely that Russian election observers would obligingly provide “evidence” of electoral fraud, as they did in Scotland.) Even if the conspiracy theories didn't get beyond the jet-fuel/steel-beams credibility threshold, Little England's low-intensity war against the EU would continue for another generation. UKIP would get MEPs elected, who would take up seats in Brussels and behave like carbon monoxide molecules in its bloodstream, taking up space and blocking its operation. Tory politicians (and perhaps Labour ones) would find that pandering to Europhobia is politically profitable. The conflict would flare up from time to time, and might again drag Britain towards the edge of the abyss.
I'm hoping for the third scenario. It could still happen, though, if polls are to be believed, is unlikely. If the polls are to be believed, the second scenario is the most likely, which means that things will fester on, albeit in a less acute state. Though recent history has shown that polls aren't what they used to be; perhaps we're entering a chaotic period of history, where assumptions no longer hold. In any case, we'll probably know between 2am and 7am.
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