The British position was that they opposed the 'Slave Trade' and used their navy to stop it.
What was actually happening was that they were using Indian indentured servitude that was slavery in all but name as forced labor while denying other countries access to that cheap agricultural labor source.
Britain didn't have to force their way to access to Southern cotton which was a big part of the Southern strategy, they had other sources.
I've said for a long time that without the Union interdicting Southern Cotton going to European markets, the only way a foreign company could compete would be to use slaves themselves.