Except it was the British Empire that persecuted slavery and brought to an end the slave trade far in advance of other nations.
What people forget is that the norms of the past are not the same as the norms of today. At one time, slavery was normal across vast swathes of the planet. That it isn't today, is in part due to a moral revolution that took place in Britain and resulted in a policy change by the British empire - despite its own commercial interests - to abolish slavery and the trade in slaves, even while nearly all other countries in the world at the time were totally fine with it.
What people forget is that the norms of the past are not the same as the norms of today. At one time, slavery was normal across vast swathes of the planet. That it isn't today, is in part due to a moral revolution that took place in Britain and resulted in a policy change by the British empire - despite its own commercial interests - to abolish slavery and the trade in slaves, even while nearly all other countries in the world at the time were totally fine with it.