Why anti-slavery Britain could still fight over Trent

Would British anti-slavery opinion have prevented a war that indirectly helped the Confederacy?

Not necessarily; Britain could oppose slavery yet still fight to defend its flag, Royal Mail, and maritime sovereignty after the Union's boarding.

British opinion contained strong hostility to slavery and little appetite for a war fought on behalf of the Confederacy. Contemporary press reaction nevertheless treated the boarding as an insult to Britain and its maritime position. The political case for action would be defence of the flag, Royal Mail, and international precedent rather than endorsement of Confederate society. That distinction allowed anti-slavery Britons to condemn the seizure while remaining suspicious of the South. A Trent war would therefore be framed as a response to Union aggression, with any Confederate benefit arising from wartime expediency.