Great Britain benefited greatly from slavery during the time of the trans-Atlantic slave trade. Let us look at two main ways in which it benefited.
First, it benefited from the labor of the slaves. There were very few slaves in Britain itself, but the economy of some parts of the empire were based on slavery. This was particularly true of the sugar islands of the West Indies. Britain benefited greatly from the income made by the slave-owning planters on those islands.
Second, it benefitted from transporting the slaves. British shipping was at that time deeply involved in the slave trade. British ships carried most of the slaves not only to the British colonies but also to the colonies of Spain and Portugal. This was a lucrative business.
In these ways, Britain benefited from transporting the slaves and from their labor.