Huh, I have convinced myself that I was completely wrong when I found out that the Chinese Emperor Yongzheng outlaws slavery in the 1720s. So the British do not get credit. However, you still need to prove that slavery was not the "statistical norm" at the time of the Transatlantic Slave Trade (in the 1700s). The primary reason are all the Muslim empires at the time. Slavery is very much permitted under Muslim law, and so slavery was widespread in north Africa, the Ottoman Empire, India, and Indonesia, and even among non-Muslims in Africa; with all those peoples still practicing slavery in the 1700s, I dunno how you can plausibly claim it was not the "statistical norm" in the 1700s. The degree to which it might not have been is entirely due to the Chinese having just outlawed it and Europeans having stopped enslaving themselves centuries earlier.