I've seen several respondents make this statement, obliquely, and my one "real" history book (English military analysis from 1872--1890) makes the same claim. Neither really explain why: the history book says British commanders were enamored with the elan and old school chivalry, and romanticism, of the South. However, romantic ideals seem to be a terrible way to conduct war.
During the buildup of the war, the South seemed to have many advantage. It had a near monopoly on the cotton trade with Europe, which meant that they could almost certainly leverage Britain and other European nations into fighting for them (remember, the Revolutionary War was won with French backing). However, when the war actually started there was a surplus of cotton in Britain, so they could make do with alternative trade (with places like Egypt) without having to go through the trouble of a war.
The South was also seen to have the moral high ground, since it's rebellion likened that of the original Revolutionary War. And yes, the Civil War was about slavery but emancipation didn't happen until halfway through the war, and it did have political ramifications.
The South also had what was seen as superior military generals: famous names like Stonewall Jackson and Robert E. Lee were on their side.
Edit: sorry, for sourcing, I'm just taking this from my highschool AP US history textbook. If you'd like to know, it's "The American Pageant," Fourteenth Edition, by David M. Kennedy, Lizabeth Cohen, and Thomas A. Bailey.