"During the eighteenth century, 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. This is a fact often forgotten by those who regularly cite Britain's prime role in the abolition of the slave trade." From this site.

No doubt there are uncertainties about those kind of statistics. There always are. But the major role played by British based slave-traders is beyond question.