How about a POD which begins before the war but culminates with a WW1 Event.
Let’s say that there are deliberate Socialist & Marxist agitators in the UK, many more than the authorities realise (and tolerate to some extent) who see the growing political tensions in Europe as the perfect time to launch the revolution. They are very well organised and sponsor literacy programmes under the guise of left-wing leaning churches to ensure that the generation that will come of age post Victoria are educated and capable of good decision making.
Fast forward to the July crisis where it looks like war in inevitable following the demands placed on the Serbian government. These agitators decide that if war is declared and the BEF is mobilised, that they encourage as many of their people to volunteer for the army as possible, to learn military tactics and to spread the word throughout the armed forces where traditionally people from the length and breadth of the United Kingdom come together for perhaps the only time in their lives before going back to their towns and cities of birth.
Germany invades Belgium, war is declared and there is a surge of volunteers to fight the Central Powers. The BEF is all but annihilated checking the German advance and so the volunteers who haven’t really completed basic training are rushed over to France in order to plug the gap. The Germans are halted at the Marne and slowly pushed back. In the trenches as the autumnal months turn to winter, the soldiers begin to tell each other stories and the best are of a vision for a post war Britain where the workers own the means of production whilst the upper and middle classes are cast aside, no longer feathering their own nests. There are things like unlimited hot water, electricity, education, pensions etc which really appeals to the average working man.
The Christmas truce rolls around and stories filter back from France over telephones and telegraphs that soldiers have laid down their arms in comaderie with their continental brethren, spontaneous football games occurring and rations traded. The socialists/agitators are able to get a message through sympathetic radiomen – one word “Go”.
As Xmas Day become Boxing Day, there are reports of mutinies in the trenches, soldiers en masse are refusing to fight, claiming that the UK Government has betrayed the working classes, forcing them to fight in this bourgeois war of empires. It was never about the Belgian people, it was about Imperialism, plain and simple. The Germans are in the dark over this and don’t take advantage as yet but the French troops are in a similar position, left wing sympathies being more prevalent in the third Republic. The soldiers that refuse to bear arms outnumber those who will carry on and embarrassingly the British contribution falls apart (which is still less than the French efforts.
So one could have a couple of hundred thousand armed troops unwilling to fight for the UK but willing to bring the revolution to the UK. Plus there would be a substantively sympathetic population who wouldn’t want to see harm to come to their fellow Brits. If they get home…are we talking insurrection on a massive scale? That’d be the worst case scenario afaik.