>>17767888
>Support the CSA in the civil war for example.
There is very little the British could have done that would not have run the risk of backfiring spectacularly.
In 1860 the British Royal Navy was already stretched thin just trying to cover all the routes between the UK and their farther colonies like India. On paper they could challenge the US Union navy, but in practice moving enough ships to actually challenge the US continental blockade would leave the UK colonies vulnerable to all their other rivals.
On land the British disadvantage would have been absolutely disastrous. In the Crimean War they had only been able deploy 35,000 men at the Siege of Sevastopol. Which is nothing compared to even a medium sized US Union Army of 50,000 men. Considering the French army in Mexico only numbered around 38,500 men it's unlikely that the British could have brought over more.
At a medium sized battle like Shiloh both sides suffered losses (killed, wounded, captured) of around 20%. At a really bloody battle like Antietam Confederate losses were around 33%. Those kind of losses over just a handful of battles would absolutely wreak a British Expeditionary Corp. Even worse the photographs of heaps and heaps of thousands dead British soldiers would eventually cause the collapse of the British government back in London.