This is a fascinating question on how the war was perceived internationally--what did other countries think caused the war?

Unfortunately I don't know off the top of my head and I don't have any good books on the topic nearby. All I can do right now is point you to this guide from the Illustrated London News which compiles that paper's coverage of the Civil War. It's just one paper from one country, but it's also a fun look into external interpretations of the conflict.

I particularly liked this clip from an article printed on Jan 12, 1861 (in the middle of the secession crisis, before any hostilities broke out):

No doubt the triumphant attitude of the North, consequent upon the recent Presidential election, may have caused the hot blood of the South to kindle into a ferment, and the proverbial strife of alienated brethren may be imminent. But may it not be suggested that the Southerns have laid too much stress on the election of Mr. Lincoln to the Presidency? and may they not have confounded the angry sensations attendant on a political defeat in his person into a political and social revolution, the advent of which the circumstances do not seem to us by any means to warrant? The Slave States appear to assume that a positive crusade against "the domestic institution" is, with a view to its ultimate extinction, about to commence. As far as we have been able to understand the situation consequent on the choice of Mr. Lincoln to fill the Presidential chair, the policy which he proposes to inaugurate is not necessarily aggressive. No one supposes, at least those who are calm lookers-on do not, that an attempt is about to be made to abolish slavery altogether in America. Unhappily, we fear that such a consummation is still far distant. All that is probably intended is to give every moral discouragement to the increase of slavery, and to pave the way for some safe, gradual, but somewhat remote removal of such an anomaly from the midst of the nation which boasts to be the freest in the world.

Here the paper's referring not just to slavery as an overarching cause but also disunion (sectional divisions exacerbated by Lincoln'd election) as an immediate catalyst for secession. That aligns really well with a lot of historians' discussions of the war's causes today.

England certainly knew what the causes were. England was in the morally questionable situation of being a huge importer of cotton from the Southern U.S., while at the same time acting as the chief World Police Force in fighting the international slave traffic. It has been observed since the 1820's or so that the slavery in the South had been on the decline until the invention of the cotton gin, at which time cotton became the 'king' crop. England would buy cotton at a relatively cheap cost, mill it into fabrics, then sell the manufactured good back to the rest of the world (including the US). As cotton became critical to the US South's economy, there arose a much more strident and belligerent defense of slavery in the US South. This would not have been necessary or even possible without England buying the cotton crop.

Sentimentally, many in the English government (even possibly Queen Victoria herself) were sympathetic to the Southern cause and they had been waiting and expecting the US to break apart into sections since the days of the Revolutionary War. Lincoln played his trump card - abolition - as a way of effectively forcing the English government to stand back and keep away from directly backing the South. England certainly deserves a lion's share of credit for fighting the slave trade, even enlisting the cooperation of some US administrations to send warships to Africa to fight the trade. The English government would have lost credibility in their fight against the slave trade if they had helped the South to win the war and undo Lincoln's abolition.

PS - I guess what I am saying here is that the Civil War was caused by sectionalism and slavery - and that England understood this fully.