An alternate challenge: with the loss of it's colonies and economic decline, Spain was seen OTL as a has-been power by the mid-1800s. With a POD after 1700, reduce the UK and France to a similar international status by no later than 1850...
Bruce
Here goes:
The British manage to win the American Revolutionary War, defeating France and the Netherlands in the process. The American colonies are Not Happy, and so the British have to maintain a large army in North America to keep them from attempting Round 2. This army is rather expensive, which means much higher taxes in Britain proper. This pisses off a lot of poor people in Britain. They don;t like the higher taxes and the fact that a worrying number of their young sons are being drafted to keep the Americans down. The French are hardly better off, as they have lost yet another war against the British and have to rebuild their navy yet again.
The French Revolution breaks out more or less on schedule. Britain tries to intervene, only to have a French-style revolution break out in Britain itself. London and most of southern England are lost to the revolutionaries, while the monarchy survives in York and northern England. Most of the British army in North America and Ireland is withdrawn to combat the revolution, and the Americans and Irish smell blood. They decide to jump on the revolutionary bandwagon, and rebel. They are not very successful initially, but continually grow in strength while the forces facing them are steadily whittled down to fight the revolution at home. Napoleon gets killed in "a whiff of grapeshot" gone wrong, and the French Revolution gets crushed by a joint Austro-Prussian force around 1800. They give France a fairly light peace, since the British revolution has not been put down yet. The British Revolution is finally crushed by sheer numbers. However, the monarchy is forced to give independence to the United States of America, and Ireland. The British and French take a sharp turn towards the reactionary in response to their devastating losses.
Around 1830, another round of revolutions rocks Germany and Italy. Nationalists rise up for a united, liberal Germany and Italy. The Prussian king is crowned king of a united Germany, while the Hapsburgs collapse and flee to Russia. The Russians and French intervene, but only manage to force a draw with the new Empire of Germany. The Hapsburgs are restored with Russian bayonets, while the new German Empire is left to consolidate control over the OTL German Empire. A new Kingdom of Italy is formed, under the House of Savoy.
Unfortunately for the Hapsburgs, Russia suffers a succession crisis in 1840ish, and the troops propping up the Hapsburgs are withdrawn to fight the a growing civil war in Russia. The Hapsburgs promptly fall in a revolution, with the German parts joining Germany and the rest being spun off as independent kingdoms. France attempts to intervene to restore the status quo, but is beaten by an Italo-German alliance. France promptly collapses in yet another revolution, with Germany and Italy watching contentedly.
Around the same time as the fall of the Hapsburgs, a massive rebellion breaks out in British India. They are forced to throw all of their strength into keeping control over India, leaving them unable to intervene in the destruction of France. They eventually manage to restore order in India, but they are too late to save France. The British are locked in a low-level guerrilla war in India, steadily draining their resources.
Meanwhile, the new powers of Germany and Italy continue their rise. The United States is growing nicely as well, eventually becoming stable after a few coups. They beat the Mexicans in a war, and now own all of North America north of the Rio Grande.
I think that's enough Brit and France-screw for today. I don't think this is too plausible, but it works. My biggest issue is the first revolution is Britain, buy everything after that is fairly plausible with butterflies.