A European Unification movement begins to try and resolve the crisis of the economic shitstorm, pushed forward by the French and Germans. One of the two aforementioned powers takes charge, UK says "fuck this" and opts out. Britain starts having shit flung at it by mainland Europe. Meanwhile, Europe starts throwing more and more weight around elsewhere, such as the Middle East. Embargoes turn into policing actions turn into more aggressive steps, and the first world powers that don't step in line start facing sanctions of their own. In light of the weakened economic position, bullying people into line becomes the order of the day to try and keep the ship from sinking. Inevitably someone will have their toes trod on, and a detente situation arises from the increased tensions. Ultimately it boils down to Europe vs Russia, because Russia has a newly united gaggle of tyrants looking to make more elbow room, and Russia doesn't want to dance to their tune. China sees an opportunity to expand and gain more power by tag teaming on Russia with Europe, and the UK aids Russia because they share the same struggle. From there, the US is roped in to preserve the UK and to try and knife China for all the cyberattack bullshit they keep trying to pull to steal US government and corporate secrets. Inevitably this turns into a full on conflict.
Unfortunately, the battlegrounds are predominantly fought in Eastern Europe, the Middle East, and the bordering territories between China and Russia, along with a combined naval and aerial embargo of the UK. This ultimately means somebody, when the war starts turning sour on their end, will begin using the heavy ordnance indiscriminately to prevent defeat, as the lands being ruined are not their own. Some territories in Eastern Europe get quite literally nuked, which brings everybody to a state of near suicidal readiness. If it was a missile that went off, God help us all. If not, diplomacy steps in as people start demanding the others to stand down. More nukes go off, as retaliatory strikes occur. From here, either a full blown nuclear holocaust happens, or more likely threat of such forces both sides to back down, leading to a new cold war.
Incredibly unlikely and requiring numerous leaps of assumption on my part, but this is what I can come up with.