Here is something I added to a previous thread where someone made the claim that if America had never broke away from Great Britain slavery would have ended 30 years sooner.

Would slavery really have ended here 30 years sooner, and without the upheaval, if there had never been a break with Great Britain. Perhaps, but remember that for 160 years slavery in North America WAS a British institution. It would not have been so easy to abolish slavery in 1834 if The United Kingdom contained an area 20 times the size of England whose entire economy was based on slave labor. Even after 1834 Great Britain was complicit in the American institution of slavery. The British textile industry, a major segment of it's economy, relied heavily on the cotton being exported from the American south, providing slaveowners with their biggest market.
   During the American Civil War there was even some fear in Washington that Britain might use it's navy to break the Northern blockade of the South which had cut off that supply of cotton, thus strangling the Southern economy and cutting them off from manufactured goods and raw materials they desperately needed. That would have neutralized the North's greatest advantage. Actually there was never any real threat of this happening, probably because of emerging cotton cultivation in other parts of the British Empire.
Nevertheless, there was strong sympathy for the Confederate States in Britain and The Emancipation Proclamation was aimed at swaying British public opinion as much as for any other reason.