You're of course only talking about local slavery in the UK. They were among the primary slave traders outside of their home territory. The British Empire and its prominent subjects traded in millions of slaves between 1400 and 1800. Centuries of profit from that slave trade helped fuel investment into industrialization across the UK.
This is a common escape for European apologists of slavery (whether in North or South America). Europe has largely gotten to avoid the consequences of their slave trade (they properly owe many trillions of dollars in reparations). The US history of slavery has nothing on the scale of European slavery - 12+ million slaves across the Americas (and that's merely the first generation number).