Legislation, by definition, involves coercion, as in the use of government force to compel compliance. Non-coercion would mean the use of persuasion to get people to voluntarily stop.
Also, England had no significant number of slaves IN ENGLAND where the voters were. The legislation also paid off the slave owners in Jamaica and elsewhere for the price of their slaves.
“Non-coercion would mean the use of persuasion to get people to voluntarily stop.”
I presume you are in favour of government regulation? As far as I can see, there is nothing compelling a jurisdiction to permit the sale, distribution and operation of pornography should the people choose to bar it.
The same way there is nothing compelling a jurisdiction to permit the sale of alcohol.