Digital Collections of Primary Sources - Enslavement and Abolition:
Centre for the Study of the Legacies of British Slavery Online Database – this massive project documents the history of enslavement in the British empire. There are detailed records for slave-holding estates, enslavers, and more. Also, crucially, this database documents the “Slave Compensation Commission” – this was a British government commission which paid out £20 million pounds to 46,000 enslavers upon the abolition of slavery in the British empire in 1833 in exchange for their loss of “property.” https://www.ucl.ac.uk/lbs/
See also their Documents of Interest Page https://www.ucl.ac.uk/lbs/documents/
Read an overview of this database here https://www.historyworkshop.org.uk/the-legacies-of-british-slave-ownership/
See the related project Western Australian Legacies of British Slavery. This academic project seeks to document how the legacies of enslavement and financial compensation paid out to enslavers by the British government shaped the settlement of Western Australia. *This project uses primary sources to inform its analysis, but isn’t a database of primary sources. https://australian-legacies-slavery.org/
Thomas Leyland Company Account Books, from the 1780s, via UMichigan. Records of two British ships involved in the transatlantic slave trade. https://quod.lib.umich.edu/l/leylandt/
Slave Voyages, a project managed by academics that compiles a database of slave voyages across the Atlantic, including thousands taken on British ships. Also has good background on the slave trade, and visualizations of the data. https://www.slavevoyages.org/
Primary Sources via the UK Parliament related to the trade in enslaved people and abolition movement https://www.parliament.uk/about/living-heritage/transformingsociety/tradeindustry/slavetrade/from-the-parliamentary-collections/
Peoples of the Historical Slave Trade project, contains more than just British history, search by place, person, events https://enslaved.org/about/