Original sentence: During the eighteenth century however, when the slave trade accounted for the transport of about 6 million Africans, Britain was responsible for almost 2.5 million of them.For two hundred years, 1440-1640, Portugal had a quasi-monopoly on the export of slaves from Africa
Edited sentence: (A fact often forgotten by those who regularly cite Britain's prime role in the abolition of the slave trade.During the eighteenth century however, when the slave trade accounted for the transport of a staggering 6 million Africans, Britain was the worst transgressor - responsible for almost 2.5 million.For two hundred years, 1440-1640, Portugal had a monopoly on the export of slaves from Africa.It is notable that they were also the last European country to abolish the institution - although, like France, it still continued to work former slaves as contract labourers, which they called libertos or engagés à temps