Maybe/probably after the war of 1812 but Britain didn't abolish slavery when it had the US.
When it abolished slavery, Britain (proper) didn't have slavery and had lost the only colony with significant slavery, so abolishing slavery throughout the empire didn't cost much.
Like I said, the US was willing to go to internal war to abolish slavery. Not many countries were.
It's one thing to do good when it doesn't cost much, it's quite another to do said good when it costs a lot.
When it abolished slavery, Britain (proper) didn't have slavery and had lost the only colony with significant slavery, so abolishing slavery throughout the empire didn't cost much.
Like I said, the US was willing to go to internal war to abolish slavery. Not many countries were.
It's one thing to do good when it doesn't cost much, it's quite another to do said good when it costs a lot.