1. The U.S. had fought two major wars with the British, the only two major wars we had fought. The Mexican-American War not being a threat to our survival. Thus, the British considered us, and we considered them, to be high on the list of potential enemies.
2. We were an economic and naval challenger to Britain. Intelligent leaders in Britain would have realized that by 1860, we were actually eclipsing them, and becoming the most powerful nation on Earth.
3. There were still territorial issues between the UK and the US. Although not mainly resolved by 1860, they were still fresh in everyone’s minds.
The normal inclination would have been to promote the weakening of the United States by encouraging civil war. This is what they did.
The involvement of the Royal Navy would have been helpful for the South, but British troops on U.S. soil would have had a galvanizing effect in the North (and negative effect in the South) far more detrimental to the South's cause than any military benefit from their participation.
The British public was very antislavery, which would have created problems for them at home.
That said, no one should discount the effect of commerce and trade. Britain needed raw cotton for its mills and the South had it. This made it well worth encouraging the South to maintain those relationships, but apparently not a strong enough motive to send warships to break through the naval blockade.
The Troy landed gentry were Pro-Southern since they viewed the Southern landed “aristocracy” as their kin. Just as the South had their battles against the Industrial North the Tories were fighting the middle/commercial classes represented by the Liberal coalition.
One of the reasons the British did not try to break the Union blockade was because the British wanted to have the option of using blockade if they were involved in a war.
I agree. The whole basis for British involvement in WWII was to knock Germany down a peg. The British never did and never would endure a rival on the open seas.