Not wanting to understate in any way the evils of slavery, but it should be remembered that during and after Britain's involvement in the slave trade, poor British people were being treated worse than slaves.
Not having any intrinsic value, they could be paid a wage below subsistance and worked to death. There were plenty more paupers to replace them. The orphans were purchased from the parish and put to work for cruelly long hours and again worked to death. Few reached adulthood.