How about this for a scenario:
Call it: "for want of a harvest".
PoD: the harvests in the UK during the 1917 and 1918 are very poor.
As a result, imports need to be increased sharply. Even so, it's not enough. Britain and her allies can just about feed the troops on the front, but the civilian population of Britain itself is reduced to starvation (as in, the middle classes are maybe 50-100 calories short of a healthy daily intake, working class is worse, upper class eating sufficiently but feeling the pinch of high food prices - not enough to kill many outright, but enough to kill in combination with months of hard war work and enough to piss alot of people off). Battles go much as OTL, with Britain going deeper into debt to the US to fund the war effort and buy food to keep everything from melting down at home. USA enters war on schedule and Germany defeated roughly on schedule.
Troops come home to find Britain in miserable state, their sufficient war-rations being replaced by insufficient civilian rations if they find work and thin broth from soup kitchens if they can't (and many can't). Ex-soldiers inject a thread of militancy into the by now powerful trade union movement that has been leading the protests to food, war and inequality. The government gets more heavy handed as rage builds and fear of Bolshevism grows.
The food situation starts to improve, but too late, discontent is still near boiling point, people are sick, half starved and desperate and the hope that a sufficient food supply brings makes the prospect of a new "war famine" all the more fearsome.
Eventually, Ireland reaches breaking point, with the IRA redoubling its efforts there and rumors flying in Great Britain that the army will be going over to Ireland in force (the rumors of course are untrue, the Government being well aware of the risks of sending over war-weary regulars to fight). On top of the demoralization of months of breaking strikes by former comrades, the army is close to mutiny. The public, getting wind of the recruitment drive for Black-and-Tans and fearing that the new "war in Ireland" will result in less food as everything is spent supporting an army over in Ireland. The Unions fear that the Black-and-Tans being recruited will in fact not be used on Ireland, but used closer to home to purge the working class with fire and blood.
Around this time, the treaty of Versailles is signed. The more desperate British position has resulted in a harsher treaty, with more reparations demanded of Germany. Unfortunately, this backfires on the British as the increased exactions from Germany, particularly in coal, drives down the prices British exports can command. Further, since Germany has less money to pay for imports, her trade with Britain crashes. Unemployment starts climbing in the factories and the coal mines.
Ironically, the people blame the government for not being MORE harsh on Germany, rumors circulate of Lloyd-George being secretly pro-German.
A general strike is called in early/late 1920. It will be the start of the British Revolution.
The Black-and-Tans, with many units successfully infiltrated by Trade Unionists, are mobilized early to deal with the massive strike, just as the Unionists had feared. As many units betray the government and join the revolutionaries' march on London, the dangerously mutinous army is called upon. The army proves even more "red" than the Black-and-Tans. Fighting between loyalists and revolutionary forces breaks out across the country, but no unit retains enough cohesion to stop the march on London.
The government decides to evacuate the Royal family. Parliament itself is in anarchy, the anti-war liberals, many of whom have been radicalized by the grim years of "Lloyd George's tyranny" trying to bring down the government. However, lines between radical and loyalist MPs are not clear-cut, with some former anti-war liberals and even labour party members being loyalists and some pro-war liberals and conservatives being radicals. As such, the hasty plot cannot be kept secret. The government, fearing Syndicalist infiltrators and traitors within parliament itself, panicked soldiers are sent to "detain" certain MPs. Some resist. Some are martyred.
As the rebels draw nearer to London, Lloyd George and most of the government decides to follow the Royal family and evacuate to "preserve the Empire from Bolshevism".
Rebels take London. The radio spreads the news of Lloyd George's "callous butchery of sitting members of parliament". Loyalist morale in Great Britain and Ireland plummets. Army units regularly surrendering or switching sides. Lloyd-George government in France orders the RN to prepare a blockade of the British Isles. Mutinies spread across RN as men and even a few officers rebel at the idea of starving their own families.
The RAF is the only service not to see rebellion and disloyalty on a significant scale, with many pilots and even a few support crew managing to withdraw to France.
Major fighting on the islands stops. Upper classes escaping however they can, for fear of the coming "Bolshevist terror". IRA and Syndicalists victorious. A constitutional conference will eventually establish a "Federal Republic of Great Britain and Ireland".
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I think I will leave it there for now... What do people think? Plausible?
I am rather curious what relations would be like between the FRGBI and the Empire (I suspect that urban intellectuals in India would love the new Syndicalist/Sinn Fein republic, the princely states would fear greatly). And where would the remnants of the British establishment, RAF and RN set up their new center of Empire? Canada I would guess...
And what would relations with the Soviet Union be like? On the one hand, the Soviets dreamed of revolutions in the West, and here one is. On the other hand, the British Revolution is a Syndicalist one, and therefore a strong ideological opponent. I can see pragmatism pushing the FRGBI and the USSR to cooperate - I can also see ideology making them worse enemies than OTL's USSR and UK were...
And what effect would this have on leftist movements elsewhere in the world? I can see the British Revolution fueling a huge red scare. I can also see many Socialist and Syndicalist movements who disliked the Bolsheviks aligning with the FRGBI, leading to those political forces being revitalized in their struggles against Bolshevism.
fasquardon