Answer:
Before the slave-trade was banned in the 1800's, the British used slaves for their plantations in the Caribbean and the Americas (before the American revolution). In the Caribbean, slaves were used to produce sugar and molasses. In North America, slaves were used to produce cotton.
Slaves were also used as house servants or construction workers, but these cases were a minority.
Slavery fueled much of the Britain's economy. It was an extremely lucrative business, as it meant free labor for highly valued exports.
Sameer