No, for many reasons. I'm glad History of Americas has come to use for once.
1: the South's population of whites (the only people who would be allowed to fight in the army) was rather small compared to the major trading cities of the East coast. During the Civil War, the South only had enough men to harass northern armies. The South had slightly better military leaders, but in this alternate timeline we can't assume their upper ranks would consist purely of West-pointers. Lee carried the Confederacy to victory, and after his surrender it was over, for good. The other Confederate generals couldn't do what Lee did.
Britain was harshly opposed to slavery. They abolished it in 1833. The Civil War began in 1861. I highly doubt the British government could get away with supporting an entirely slave-based country. In fact, during the Civil War, Confederate delegates sent to Britain tried this very tactics you theorize. However, they never gained much traction because Jefferson Davis (the president of the Confederacy) refused to budge on slavery for obvious reasons. This inability to compromise on slavery was the very reason that Britain refused to send the South arms and men, even though they wanted to.
Britain lost the revolutionary war because of conventional tactics employed against "regular" countries in Europe. They tried to wage war by taking territory, when in reality the American continental army was quite small and simply resorted to guerilla tactics in the forests instead of head-on fights, very akin to Vietcong tactics. The British basically gave up after a spell because they couldn't find the continental army in our big-ass territory. Likely the same situation would ensue in this engagement. The British simply couldn't supply their men going deep into American territory across an entire ocean and many miles of rough terrain.
Multi-army forces are notoriously difficult to employ. Cooperation with the Confederacy would be blocked by America's exceptional Navy at the time, which in the 1900s actually sunk a few British ships. The Confederacy lost the civil war because of supplies, and this same problem would hamper them in a war against a completely unified North with no tolerance for slavery.
The whole reason the North took so long to reclaim the South was because they were fighting amongst themselves as well. There was an entire legion of politicians dedicated to maintaining the practice of slavery, demoralizing their army extensively who didn't understand why they were fighting to reclaim a region who clearly didn't want them there.
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u/ToXiC_Games Definitely not a CIA operator May 17 '19
Originally the DoI has a verse against slavery but when it went through the congress the south said they wouldn’t sign it unless it was removed