A mark of just how complex the attitudes of Britons toward the US Civil War was:

From Battle Cry of Freedom, p 552

"...several liberals and even radicals were attracted to the South's fight for self-determination.   Many Englishmen had cheered the Greek fight for independence or the struggle of Hungary and Italian states to throw off Hapsburg rule. Some viewed the South's revolution against Yankee overlordship in a similar light".


This is why the Emancipation Proclamation was so important.

"To accept the notion that the South fought for independence rather than slavery required considerable mental legerdemain. But so long as the North did NOT fight for freedom, many Britons could see no moral superiority in the Union cause".    Battle Cry p 553.

Looked at from the this perspective, a good argument can be made that the most dangerous time for the Union cause was early in the war--especially at the time of the Trent affair, when hotheads on both sides of the Atlantic were in fact eager to go to war against each other.