As would have been relevant to a thread from a week or so ago asking about Russian's support of the Union during the Civil War, it was Russian diplomats who informed the Union that British, French, and much of conservative European opinion was favorable to the recognition of the Confederacy. The reason given for the British desire to see the United States split in two is the large landmass and population of the US being under a single government. They feared industrial competition from the unified country in the future, preferring something like a Balkanization of the states in a way that created a balance of power that would insulate Europe from any challenges to their general hegemony.

Another equal possibility is something I could have sworn I've read, but can't find at the moment. I recall seeing that the British government chartered the SS Great Eastern to transport troops to Canada at the outbreak of the Civil War. The reason was supposedly that they were worried about raids by Irish Fenians from the US while the government was distracted in the south. Perhaps support of the South among British subjects was as much attributable to a desire to see a balance of power set up in the States, as it was to animosity towards radical Irish elements in the North. The prevalence of Fenian sentiment in the North can be seen in SP Heintzelman's diary when he relates that General Hooker was an active supporter of the Fenian movements.