It should be understood that the british empire was at the peak of economic dependence on the raw material coming from the "new world" at the onset of the civil war; in particular, cotton exports for consumption by british textile mills [1].

> By the late 1850s, cotton grown in the United States accounted for 77 percent of the 800 million pounds of cotton consumed in Britain. It also accounted for 90 percent of the 192 million pounds used in France, 60 percent of the 115 million pounds spun in the Zollverein, and 92 percent of the 102 million pounds manufactured in Russia.

One reason that there was support for the confederacy was the fear that the outcome of the war would lead to the end of access to abundant and cheap cotton (due to export duties, end of slavery, etc.).

One of the outcomes of the US Civil War was that the British Empire realized a need to "diversify" their sources, resulting in increasingly imperialistic behavior in India and Egypt, among others.

[1] https://www.theatlantic.com/business/archive/2014/12/empire-... the whole book is a fantastic peek behind the curtains of the history of global capital markets