Ok so that’s quite a fanciful claim, but just for fun here’s how it might happen.
Step 1: Scotland leaves the UK
In the 2014 Referendum on Scottish Independence there was the issue that Scotland would have to re-enter the EU as an independent country. I.e. it would have to exit the EU first and fulfill the 35 rules before re-entering. This was a bit of a ruse by the various EU leaders, including our own, as there were a number of other countries facing the same breakups e.g. Spain with Catalonia. So making the future look bleak for any breakaway factions was part of a larger, stabilising plan.
But, in 2016, once Article 50 has been triggered the UK is already out of the EU and Scotland voted against that!
So Scotland is almost compelled to have a second referendum and the EU issue has disappeared, in fact the reverse is now true. Scottish independence is phrased as “those Sassenachs are nothing to do with us”, victory is assured, and the EU welcomes them with open arms.
Step 2: Scotland trades oil (in Euro)
Scotland adopts the Euro as their currency; there’s no reason to take Sterling and plenty of emotional reasons against. They also get a large part of the oil reserves from their newly assigned North Sea territories.
But why trade oil in US Dollars? Much of Scotland’s oil trade will be with other EU countries, so a new oil exchange is formed, probably in Frankfurt or maybe Edinburgh, trading in Euro.
In the meantime Northern Ireland, which had a large Remain contingent, has a referendum to unify with Ireland and there’s a good chance of success – and the UK ceases to exist (well, the United Kingdom of England, Wales and a few small islands).
Euro finance trading is hemorrhaging from London to Frankfurt and oil income is greatly reduced. From this point on it’s inevitable – Sterling sinks, GB converts to Euro and probably rejoins the EU.
And it’s all the Brexiter’s fault. Well, ok, it’s also the fault of SNP-Remain running a much better campaign than Corbyn and Cameron managed…
Bonus: US Dollar also crashes
Ok, this one is not so likely, and would take 25 years, but here goes.
The future of the world lies in clean energy. Countries like Dubai are already building huge solar-electricity plants. Once the technology matures massive investment is made in all the equatorial countries, which includes southern EU, with high-voltage grids selling power to the north. The, now strong, Euro oil exchange market expands to cover all energy types with inward investment from various equatorial Eastern nations.
But no matter how good electric cars and rains become, there will always be plastic products and trucks needing diesel. Oil demand only reduces a little and the Arab oil producing nations want a part of this new market – they start trading some oil in Euro as well.
From here the Euro oil market just continues to grow, a side effect is that Frankfurt becomes a finance powerhouse, and the "reserve currency" value of the US dollar starts to collapse.
Silver Lining Alternative
The European Common Market (now called the European Economic Area) was actually an excellent idea, but the thoughts of “closer European Union” – i.e. a federated United States of Europe – and the increased distancing of the European Commission from the “ordinary” public is causing significant segments of the EU population to hate the EU.
So maybe a bunch of other countries get their EU Referendums in before Scotland gets the UK one. The EU basically collapses except for maybe a few core countries.
Out of this rubble a new Economic Zone is created. We’d still have to pay just as much as now (perhaps more as there’d be no rebate) and migration would still be open (it must to be to allow free trade and exchange of services). It would take 10 years to wind the clock back to pre Maastricht 1991…