There were Black people in Britain in Roman times, and there has been a continuous Black presence since 1555.
The 18th century saw a great expansion in Britain’s Black population. After the War of the Spanish Succession in 1713, British slavers dominated the infamous Atlantic slave trade. Some slaves were landed and sold at London, Liverpool or Bristol, but many Black people were brought as domestic servants by returning ship captains, colonial administrators and plantation owners.
For the English aristocracy and the newly rich, a Black page or handmaiden was an asset to be shown off as evidence of exotic wealth.
By the 1760s, there were an estimated 20,000 Blacks in London -- a city of 676,250 people. Many had attained freedom or run away from their masters. In 1772, Lord Chief Justice Mansfield’s historic decision in the case of runaway John Somerset ruled that a slave could not be deported from Britain against his or her will. This was the beginning of the end of slavery in Britain, and an encouragement to Black people and to abolitionist campaigners. The abolition of slavery was confirmed in 1806 by an Act of Parliament.
Slavery in the US did not end until 1865 when the Thirteenth Amendment to the Constitution was ratified by the legislatures of 27 of the then 36 states.
The amendment was adopted on December 6, 1865, when Georgia ratified the amendment. It was declared, in a proclamation of Secretary of State William Seward, dated December 18, 1865.
A number of states "changed their minds."
New Jersey ratified in 1866, after having rejected it in 1865. Delaware ratified in 1901, after having rejected it in 1865. Kentucky ratified in 1976, after having rejected it in 1865.
The most recent ratification occurred in 1995 in Mississippi, which was the last of the 36 states in existence in 1865, and had rejected it back then. (info from Black History Month UK, and Wikipedia)