what about pro USA neutrality (ala pre-pearl harbour USA to Britain)?
I get the feeling this is going to be quite an unpopular opinion, but British neutrality was pro-Union. Not pro-Union enough to satisfy the Union, but certainly pro-Union enough to really, really alienate the Confederacy. There are a myriad small ways in which Britain bends the rules to help the Union: letting her adhere to the Treaty of Paris which she never signed, banning both sides from bringing prizes into her ports, stopping inspections of the blockade to see if it's effective, allowing Union warships to coal at British ports, and so on. At the end of the day, even if Britain had interpreted neutrality strictly, staying neutral was always going to benefit the larger, more powerful North more than the South.
You mean kind of like OTL
Huh, maybe not that unpopular after all.
encouraging British traders to respect the blockade
Performing all the duties of neutrality toward the respective belligerent states, we may reasonably expect them not to interfere with our lawful enjoyment of its benefits.... In pursuance of this policy, the laws of the United States do not forbid their citizens to sell to either of the belligerent powers articles contraband of war or take munitions of war or soldiers on board their private ships for transportation; and although in so doing the individual citizen exposes his property or person to some of the hazards of war, his acts do not involve any breach of national neutrality nor of themselves implicate the Government. Thus, during the progress of the present war in Europe, our citizens have, without national responsibility therefor, sold gunpowder and arms to all buyers, regardless of the destination of those articles. (Franklin Pierce: "
Third Annual Message," December 31, 1855. )
not actively selling warships and supplies to the Confederacy,
Britain sold around 80,000 weapons to the Confederacy, but 436,000 to the Union. As for warships, the US Supreme Court determined in the case of the
Santissima Trinidad that
"the sending of armed vessels or of munitions of war from a neutral country to a belligerent port for sale as articles of commerce is unlawful only as it subjects the property to confiscation on capture by the other belligerent. No neutral state is bound to prohibit the exportation of contraband articles." The ships sent to the Confederacy are not armed, and thus their sale is technically legal as per the 1819 Foreign Enlistment Act. Despite this, Britain stretches the law on the Union's behalf: she would have seized the Alabama had the Queen's Advocate not gone insane at an inconvenient point, she did seize the Alexandra and the Oreto only to have the seizures overturned because of a lack of evidence, and she prevented the Laird rams from joining the Confederate navy by buying them.
You might have seen some British intervention if the new Republican administration had taken a completely different approach to foreign affairs. There's actually a reasonable amount of enthusiasm for Lincoln's election, but it drops down when he says he's got no intention of acting against slavery and dies a death when he rescinds Fremont's emancipation scheme. If Adams had been empowered to tell Palmerston on day 1 "we're not like the guys before, we believe in ending slavery but we'll have to do it gradually, we want to work with Britain and we've definitely got no interest in annexing Canada," offered them a treaty on the right of search, and apologised for their actions when/if the Trent was boarded, I can see a scenario where Britain allows Union recruitment in Britain and Ireland by mid-1862. However, you'd probably need someone less Anglophobic than Seward as Secretary of State. I'd suggest Charles Sumner, but I'm not sure what job you'd have to give Seward as a replacement.