The United States had legalized slavery for about 90 years. In those 90 years there was tremendous amount of turmoil over it and its relation to what the Founders had envisioned. It eventually ended up in a civil war where 500,000 white people died fighting over it.
The British colony's of course had slaves before the America won its independence from them. If Britain had wanted it banned from the colonies it could have done so.
What was that Talking Heads album? Oh yeah, "Stop Making Sense". ;)
If the blasted revolutionists had not had their way and taken the colonies out of the Realm, she WOULD have banned slavery decades before the US went into convulsions over it.