The slave trade began with Portuguese (and some Spanish) traders, taking mainly West African (but some Central African) slaves to the American colonies they had conquered in the 15th century. British sailors became involved in the trade in the 16th century and their involvement increased in the 18th century when the Treaty of Utrecht (1713) gave them the right to sell slaves in the Spanish Empire. The slave trade made a great deal of profit for those who sold and exchanged slaves. Therefore, they often ignored the fact it was inhuman and unfair.
At least 12 million Africans were taken to the Americas as slaves between 1532 and 1832 and at least a third of them in British ships.
For the British slave traders it was a three-legged journey called the 'triangular trade':