The University College London has compiled a database of British slave owners. Specifically, these are people who made claims to the crown for compensation when Britain outlawed slavery in most of the crown colonies in 1833. The database lists some 46,000 claims in total. The total value of the claims was about £20 million, which represented some 40% of all annual government spending. At the time, this was a massive bailout by the government.
About 3,000 of the claims were from people living in Britain. Most of the rest were primarily from plantations in the Caribbean. This is a good website to check if you potentially had ancestors who were slaves on one of the Caribbean islands under British possession in the early 1800s. Britain outlawed the slave trade in 1807, but slavery itself was not outlawed in the colonies until 1833 (the United States followed in 1865 and Brazil in 1888). Some estimates suggest that about 10% of wealthy Britons in the early 1800s were directly connected to the slave trade.
The website hosting the database is called Legacies of British Slave Ownership and it contains a wealth of background information. Access is free.