The British did aid the rebellion, to the tune of
One of the reasonings behind certain leaders joining the CSA in the ACW was the belief that European trading partners would help them as Union blockades impeded their exports. To their dismay, no Europeans came through, and the CSA fought a doomed war where they were significantly outnumbered and outsupplied. Say the Europeans DID aid the South. In what ways would they most likely send aid, and would it even be enough?
The British did aid the rebellion, to the tune of millions of dollars worth of arms and supplies, and free access for blockade runners to British ports from the West Indies north...
However, the reality is the vast majority of that was cash and carry, or trade for contraband - notably cotton - in southern ports.
As it was, the British came close to provoking and/or aiding and abetting causus belli on multiple occasions - the Saint Albans raid and the SS Chesapeake incident being the obvious ones, but far from the only ones - but were careful enough to ratchet things down when necessary; the Laird Rams incident is an obvious example of that.
Frankly, none of the Europeans had anything to gain from overt assistance to the rebellion, and much to lose...
Most notably, "anything" sent to the rebels without some sort of worthwhile
quid pro quo was one less "something" to be used at home, deployed/sent to the actual empire (British, French, etc.), or sold somewhere (Latin America, Japan, China, etc.) where there was real money to made...unlike the shinplasters the rebellion was rapidly reduced to in terms of cash, and without the risk of the USN capturing the runner.
There's
no rational reason any European power would have overtly aided the rebels and multiple reasons - economic, strategic, diplomatic, and political - none, in fact, did...
The precedent alone is reason enough for the Europeans to shy away from overt action; none of the European powers was exactly robust in terms of their own "national" consolidation at this point, much less their various and sundry colonial and imperial territories.
And they all had neighbors - in Europe, and thus much closer to home than 3,000 or more miles away, and across the bounding main - ready to caste covetous eyes on any one of multiple territories and interests that any European power that got significantly involved in the Western Hemisphere in the 1860s would have exposed...
The rebels were among the most self-deluded individuals on the face of the earth in 1861-65; there's a reason Margaret Mitchell gave Rhett Butler the line about "all we have is arrogance."
Best,