1862: Trent Affair leads to the Royal Army and Royal Navy joining the side of the Confederate States. [This is pretty weak historically and story telling wise, but for a trashy beach/airplane/travel read it is better then nothing]
A decent read, but with all due respect to CanoeRebel, not very compos mentos. Palmerston did indeed ask, after Trent, and got brought up short by his Ministers.
Clarendon (and then Shelburne) was pretty specific about the lack of deployable troops. All they could do was perhaps send 20,000 mixed regulars and territorials to Canada. That was about it.
There weren't no regulars available to go kiting about in the US South. There weren't no regulars available to do jack all, anywhere. The Turkish war was done, the troops were home, and the next time they had to do anything seriously intelligent was the Boer War, where they learned how to fight against a modern opponent.
If I was a Confederate, I would be thankful, but not very hopeful, for a British intervention.