Well for one the Trent affair by itself wasn't enough to force their hand (that's a point I make in
Wrapped in Flames, and one TFSmith makes in his TL
Burnished Rows of Steel to good effect with some well researched follow up). Another part is purely economics. Though the loss of US trade wouldn't be crippling to the British, it wouldn't be a light pinch either (mind you if the blockade is broken it creates an amusing avenue for smuggling from North to South) and while Southern cotton was a sticking point between Britain and the US, the cheap wheat from the North couldn't be overlooked either. Another point of course is the damage which could have been caused to the British merchant marine by American raiders, which they would have had to spend some time tracking down.
On the diplomatic front there were other reasons as well. Though there were a number of proponents who were all gung-ho to recognize the south as late as September 1862 (Russel and Gladstone being the most notable, Palmerston telling them to knock it off in October) and even though many in Britain still saw Southern secession as inevitable as late as 1863 (with most changing their minds as 1864 rolled around) and were concerned on humanitarian grounds for the well being of both the white population and the blacks of the whole Union. However, Lincoln's passage of the Emancipation Proclamation did much to dampen (but I stress not completely) support for the South. The final nail in the coffin though was the 13th Amendment where no one in Britain could even offer the pretense of supporting the South.
All in all it would take some pretty big divergences to get the two sides to go to war. The public on both sides of the Atlantic would need to be incensed over some issue, and the two sides would have to be angry enough to intervene.
***
On the matter of wheat, see my post at
https://groups.google.com/d/msg/soc.history.what-if/zLZg6EzfPps/W5SwgLfuUKkJ where I note
Interestingly, Owsley, despite his rejection of the wheat thesis, shared
the economic determinism of his time (the original edition of *King Cotton
Diplomacy* was published in 1931). (He rejected the notion that anti-
slavery sentiment among the British working classes was responsible for
non-intervention, arguing that they were far from unanimous on the war,
and that in any event they mostly could not vote, nor was there much
danger of insurrection.) He wrote that "One must admit the correctness of
the principle laid down by the economic interpretation group of
historians, namely that in order to counteract one economic impulse
another stronger economic motive is necessary. But it is difficult to see
that wheat was a strong element in the economic impulse which counteracted
the King Cotton impulse. It is proposed to substitute a much more
sinister term for wheat--'war profits.'" Here, obviously, Owsley had the
experience of the Great War in mind (and indeed says so). Anyway, he
ticks off (pp. 549-57 of the second, 1959, edition) a list of economic
interests that profited by a continuation of the war:
"Perhaps the most surprising of the war profits was in the cotton industry
itself. [Prior to the war] the warehouses of India, China, and of
England...had a surplus... The mills were already beginning to slow down
before the war...Then the Civil War came as if in answer to a prayer and
cut off the supply of cheap cotton. The price of raw cotton rose from
fourteen cents to sixty, and as time passed the surplus manufactured goods
followed...In the meantime, the larger and well-financed mills continued
to manufacture goods and hold against the rising markets. These larger
mills...not merely made a profit out of this vast surplus of cheap pre-war
goods, but managed a neat profit on their output over the four years of
war. The only people who went down were the small mill-owners and the
cotton operatives. They lost all they had. But the industry was saved
from one of the worst panics in history, and impending ruin turned into
undreamt-of profits....There is another phase of the cotton profits which
must not be overlooked, namely the development of India as a rival source
of raw cotton...The next great sources of profits are closely related to
the cotton industry--the profits which were reaped from the linen and
woolen industries, the old rivals of cotton...Another business which
prospered mightily during war conditions was the munitions industry...Nor
does [this figure] include the sale of ships and steamers to the
Confederacy or the building of steamers for English blockade-runners.
This last item is of great importance, for it stimulated very greatly the
shipbuilding industry...But the greatest profit of all, one which was so
enormous it cannot be measured in dollars and cents, was made possible by
the complete destruction of the American merchant marine directly or
indirectly by the Confederate privateers and cruisers...We see, therefore,
that England far from being hag-ridden by poverty during the American
Civil War made enormous material profits." Owsley quotes *The Times*
(January 7, 1864) as saying:
"Outside of Lancashire it would not be known that anything had occurred to
injure the national trade. That is the most extraordinary and surprising
incident of the story. An industry which we conceived to be essential to
our commercial greatness has been utterly prostrated, without affecting
the greatness in any perceptible degree. We are as busy, as rich, and as
fortunate in our trade as if the American war had never broken out, and
our trade with the states had never been disturbed. Cotton was no king,
notwithstanding the prerogatives which had been loudly claimed for him."