How long do you think it would take for the South to be independent in name only being little more than a protectorate of the US in TTL? 1900? 1920?
Alternatively, British businessmen would invest in Confederate businesses and do-gooder British philanphropists could provide a great deal of aid to the Confederacy in its economic ruin and thus make it little more than a British protectorate. Seems about as likely as Northern Businessmen investing their money in the south.
Maybe you could have a faux-war of businessmen in the Confederacy between British and US businessmen vying for control of the Confederacies resources.
Risky for the Brits as it is in the backyard of the US and meddling in the CSA risks the US meddling in Europe. Also the CSA is MUCH closer which means less transportation costs. The railroads connect right up with the US. Also the Brits seemed to consider the Western Hemisphere the American "sphere of influence". Getting too involved with the CSA would run counter to that policy.
So you don't know any economic history then? South America was British and other Europeans investment area right up to WW1, and the Monroe doctrine was underwritten by British naval power.
Britain doesn't give a shit about the US meddling in Europe as it has no capability to do so, the US also has a massive paucity of capital to apply abroad, as its a tied up in its own internal development, the US was taking massive investment from the UK right up to the turn of the century, there's no way they're competing in foreign investment markets, or even in manufactured goods markets till the 1890s or so.
The British, as a global political force in the late 1800's/early 1900's were kind of committed to weakning any rival powers that might emerge and if permitting the United States to gain control of the resources of the Confederacy, especially with the increase of rival industial manufacturers in the US, would result in the strengthening of the US and make them a more of potential rival to global economic dominance then I see no reason to suggest that the British would not be prepared to get involve themselves in the Confederacy and attempt to gain control of those resources themselves.
And the British do-gooders would get involved regardless of the risk. There is all to much money available to the upper-class in British society in this time and there isn't a shortage of people willing to donate money to a good cause. Feed the poor, clothe the hungry and so on.
with all due respect, i believe that you missed the conditions of the OP.When McClellan takes command the war is already in the endgame. Lee is shut up in the siege of Petersburg, Hood has been effectively destroyed etc.
Also, it's far more likely the CSA would have become a dependency of the UK and part of the "informal empire". US and UK business interests would likely vie for investment in the CSA.
I think OTL was a pretty realistic Confederate Timeline. A Confederate Victory Timeline should have victory in 1862 or before, by 1864 its pretty much to late.
McClellan was anti-abolition and anti-civil rights, but he'd give up the Union the day Hell froze over. He'd also be smart enough not to screw up a war that's all but won, and the primary difference is that this war ends without a 13th, 14th, or 15th Amendment, meaning Reconstruction is much more murky and horrible and slavery is never really legally abolished. McClellan would have more political problems than Lincoln but militarily he *was* a War Democrat.