This is pretty simple, actually. All you need is one spark and the 10 April 1848 Petition in London, or the Manchester Workhouse Riots, could have gone completely differently. Something as inelaborate as having some random, anonymous, encounter between Chartists protester and London police go slightly different and the whole thing would have exploded; the Chartists would have surged across the Thames, and the military would have intervened. The countryside would go up in flames, and Manchester would likely fall into Chartists hands. Things would quickly have gotten out of control just like they did on the Continent. And that's not even counting what the Irish would do, or throughout the rest of the empire.