Here's a bit of a topical one.
Britain votes 53-47 to Remain in the EU, which sparks massive amounts of violence by Leave supporters, who go in for a whole racist terror vibe which eventually develops into a messy Civil War. It is called the Second British Civil War without anybody being quite sure of when the first one was. The Anarchy? The Wars of the Roses? That time when we killed the King? Nah, that was the English Civil War. Most people tend to think it had something to do with slavery. Anyway...
After a few years of death and violence and horror, the UN finally manages to impose order, and draws up a ceasefire line based on which areas voted to Leave or Remain in the 2016 referendum. As many areas were finely balanced one way or another, this impresses nobody, and the borders created bear no relation to things like 'where the reservoirs are' or 'where the roads go to'. As such, the career of Border Guard is one of the fastest growing professions in the whole of Great Britain. The Leave areas are all under the sway of the United Kingdom of England Et Cetera, reigned over by Charles III (following the example of his namesake in having a massive War) but ruled by Boris Johnson, Michael Gove and Arron Banks - Nigel Farage having killed himself in a bunker somewhere. This is an extremely socially conservative nation with its new capital in Skegness. Scotland and most of Wales have gone independent, both EU members, with Scotland taking the achingly modern official name of 'Scotland.EU', much to the revulsion of every ambassador in the world. Meanwhile, Wales is in the awkward position of being a Socialist Republic (basically out of tradition) while most Labour-voting areas are in the United Kingdom, and the Socialist Republic is therefore ruled by a conservative coalition government which shows no sign of losing ground to actual Leftists. Merseyside, however, is full of Trots and the iron-fisted rule of Joe Anderson has kept this pocket of pro-European fervour aflame for six years, with no sign of stopping. They are reputed to be the heartland of pro-Remain militia activity due to Irish-Liverpudlian ties to the IRA.
The two major EU member states are the Workers' Republic of Remainia, ruled by President Farron (another example of a country with left-wing rhetoric in the name actually being governed by a Lib Dem-Conservative coalition, because The North Isn't Actually Just Sheffield). Remainia was hit the hardest by Forced Partition, as most communities (e.g. Tyneside) are split and silly distended spurs of land combine to produce a country that is used in High Schools to explain fractal geometry. Meanwhile, the Progressive Republic takes in most of Middle England and almost all of London, and is governed by General Dan Jarvis of the European People's Party, who led the Progressive Army to, if not victory, then at least survival, in the Civil War. The Republic was, like Remainia, slightly buggered by the ceasefire line, which cut off formerly-integral parts of their territory and made them into exclaves under siege conditions.
Finally, some small areas which were entirely surrounded by people who voted the other way are turned into Condominiums, governed equally by the EU, the UK, and the private security contractors, G4S. Norwich is occupied by the Danish Army.
Nobody talks about what happened to Northern Ireland.