There is the International Slavery Museum in Liverpool, England.
http://www.liverpoolmuseums.org.uk/ism/
There is also the Unesco Slave Trade Archives http://portal.unesco.org/ci/en/ev.php-URL_ID=8780&URL_DO=DO_TOPIC&URL_SECTION=201.html
If you would like to research collections in other archives, you could try searching the Archives Hub, a catalog of archives in the UK. I did some searching in it, and you can slaveholders' papers in collections, as well as ship manifests and other primary documents. For example, there is the Pinney Collection at the University of Bristol, papers of a family that managed a plantation in the West Indies, ranging from 1688-1850. http://archiveshub.ac.uk/
