One reason that the british did not try to break the union blockade of the south during the civil war was that

They did not fully agree with the south having slaves.  
One of the first industrialized countries that said no the slavery was England in 1853?   (Not sure of the year.)  British ships would stop slave ships and turn them around back to the ivory coast.  
Also the Union was manufacturing ironclads ships, that were superior to the British ships.