Oh for... the United States of 1810s and the United States of 1860s are two utterly and completely different beasts in ways no European can really grok due to the inherent different nature of the US to typical European countries.
The first major difference is this:
Bulgarian Population in 1810: 2,200,000
Bulgarian Population in 1860: 2,540,000
French Population in 1810: 30,000,000
French Population in 1860: 37,200,000
Italian Population in 1810: 19,600,000
Italian Population in 1860: 26,000,000
Spanish Population in 1810: 11,900,000
Spanish Population in 1860: 15,600,000
GB Population in 1810: 10,186,000
GB Population in 1860: 28,917,900
US Population in 1810: 7,239,881
US Population in 1860: 31,443,321
I want you to stop and really understand what those numbers mean. In the same period of time France saw a 24% increase in population Great Britain saw an 184% growth in population while the US saw a 334% increase in population, nearly DOUBLE that of Great Britain. The US, in 1812, had 3 million LESS people than Great Britain did, by the 1860s they had 3 million MORE. And the majority of that population growth was in the North, as that massive population boom was driven in large part by various waves of immigration to the US from various European countries that almost ALL went to the northern and northwestern states as that was where opportunities were.
Meanwhile in the same period the US had seen a massive wave of industrialization in the north that completely upended the economies of the region.
In 1810 around ~72% of the US population was farm labor, by 1860 it had dropped to ~56%. This is a dramatic transformation of the economy.
Likewise the US went from having no railroads in 1810 (in fairness, nobody had railroads in 1810, the first profitable railroad didn't open in England until 1812) to having 30,000 miles of track laid, while Great Britain had around 10,000 miles. Granted, not entirely a fair comparison, as GB's railroads served a larger percentage of the country than the US ones did due to the sheer size difference. However,
for reference, both of these countries had more rail-line laid than in any other country in Europe at the time. Likewise the first true telecommunication system, the telegraph, was
considerably more widespread in the US than in any other country, with the US having 23,000 miles of telegraph line internally compared to 2,200 miles for Great Britain (who was the highest in Europe) in the 1850s. What this means, from a practical standpoint, is that the US would have much better coordination and control of operations in North America than Britain would and be able to use their railroads to move troops around much more effectively to respond to any land attacks.
As to the most critical force in this comparison, the navies, the US Navy in the 1810s relatively new and overall small, with only 5,000 personnel and 14 ocean going ships. During the Civil War the US navy rapidly grew from
42 ships to 671.
This was considerably more ships than the Royal Navy fielded at the time (398), and most of the US ships were integrating new technology at a rapid pace while the Royal navy had considerable amounts of legacy ships that would not have been able to compete. Add in that the Royal navy would have had to remain on station across much the world, while the US Navy would be mainly focused on defending the continent and maintaining the blockade, and the numbers simply do not look good for the British, who would not be able to concentrate their forces effectively compared to the US.
So no, British direct military intervention in the US Civil War is not as much as a sure thing as you'd think. If it somehow happened early enough in the war it could force a negotiated end, but the capabilities of Britain and America were no so far apart as you seem to think they were, and the US of the 1860s was a VASTLY and completely DIFFERENT beast compared to the US of the 1810s. To even suggest that just because things went one way in 1812 that they would play out similarly in 1860 just demonstrates your utter ignorance of American history and just how dramatically the US rose and developed as a world power in the 19th century. By the 1860s the US was a top tier world power on par with any of the European empires, and superior to many of them when it came to industry and innovation. Sure we didn't do international colonialism the same way the Europeans did, but then we didn't need to, as the US was not only a massive home market for our own goods, we tended to have most raw materials we needed within our territory anyway, and so were really going out looking for luxury goods and new markets to sell to (and we were willing to sell to anyone, as we had long ago rejected the Mercantile policies that drove European colonialism).
Just... really... citing 1812 as support for something in 1860? That 50 year period is perhaps one of the most dramatic shifts in human history. That is the core years of the Industrial Revolution. That is when the Steam engine is invented and the railroads began connecting across continents in ways never before imagined; the telegraph is invented and saw the first trans-Atlantic cables laid. This was when the era of Wooden Ships and Iron Men saw it's final "Hurrah!" and firearms went predominately from smoothbore to rifled and the dramatic increase in accuracy was achieved. The revolver was invented and the groundwork laid for automatic firearms. There's been few times of such dramatic changes in 50 years... and you stumbled into arguably the biggest one in world history...
en.wikipedia.org
