This is the same premise as in the TLIAD "Centuries of Shadow" by Mumby on AH.com, though differently done: Somersett VS Stewart goes the other way in 1771 and slavery is kept legal in England, eventually resulting on a Decades of Darkness-ish, slaveocratic British Empire.
The divergence did not stop the formation of the United States, but it resulted in the Abolitionist cause to be more taken into consideration, and some of the loyalist and rebel positions to be switched, and so the USA was born an Abolitionist nation. A French Revolution happened, but had much less radical consequences, with the French king being able to make general concessions, and Napoleon never reached greatness. Spain had its own revolution in the 1810's, drawing Portugal with it and keeping some of her colonies while Bolivar still instigated a wave of independence movements. the Portuguese king was beheaded andthe Spanish one fled to America, where he kept the Old Order. Brazil messily split after a wave of rebellions against the Hispanic Federation, giving birth to the states of Rio de Janeiro, Sao Paulo, Rio Grande do Sul, Minas Gerais, Bahia, Pernambuco, Pará and Mato Grosso. Decades later, more European countries would experience a revolutionary wave, and would adopt Spartakism as the name of their political aspirations.
Like IOTL, Russia was the focus of Socialist Revolutionary thinking, but earlier and with proto-anarchist tones (they adopted the name of Nihilism, as they aspired to anihilate the old world). The result was particularily nasty in Russia, resembling a perpetual state of French revolutionary terror, but not as bad in other places (it brought efficiency and greater racial equality to the caudillistic and corrupt Andean nations, even though it didn't bring more freedom). A Moderate France left behind in the colonial race and closed in herself signed with the United States what would become the Leage of Armed Neutrality, an alliance of countries that were equally turned off by Britain and by the new revolutionary developments, and wanted to defend their sovereignity and forget about the outside world.The Portuguese speaking America experienced a mildly authoritarian pan-movement in the 20th century, but as Pan-Arabism IOTL, if failed to bring real unity (and as the EU IOTL, it brought a great degree of economic integration).
The British empire has adapted the innovations of the industrial era to a heavily stratified society, with professional castes and different status of serfdom, disliked by everyone except for her New Spanish sidekick, dominated by the trading companies and hell-bent on world domination. The Empire holds a much greater pressence in Africa and Asia, and doesn't hesitate in putting down rebellions by any means neccesary, but how long such a bloated and vertical entity can last is a matter of discussion.