Revolutions.
Or more specifically, No Reform Act of 1832 and Ernest Augustus becoming King leads to the radicalized Chartists overthrowing the Monarchy following defeats to America in the Canadian War, and the Chinese in the Opium War. Britain then became a force for Republicanism, supporting Republican Revolutionaries in the Alt-1848.
Ah. I figure that could be it given what I know of the man. Good POD.