James Relly might fairly be called the first Universalist of the modern era. A disciple of Whitfield and contemporary of the Wesleys, Relly left a now little-read corpus of work, including his provocative Union, where he identifies Jesus Christ as the captain of humanity to whom our sins are justly imputed. American readers of Union, including Judith Sargent, became the first willing hearers of one of Relly's disciples, John Murray. And, as they say, the rest is history.
I've written about James Relly's London. Poking through Google Books, I found a citation of where he was buried: the burial ground of the Maze Pond Baptist Church. (Which after-Googling was noted at the Unitarian Universalist Historical Society site all along.)
Now where's that: apart from it being in Southwark, south of the Thames from the City of London (proper) and Westminster, I can't really tell. Plus, the Baptist church moved in the nineteenth century. I doubt either building is extant, and all I hope is that the burial ground isn't under a road. There's maddeningly little about Georgian Southwark on the web!