Dear Rabbi Simon
I read that some recommend visiting the graves of tzaddikim (the Righteous) where one is not able to visit the graveside of a departed family member on a yahrtzeit. This may seem like a strange question, but it is a practical one (as my sister’s yahrtzeit is approaching and she is buried overseas): Who are regarded as the greatest tzaddikim who are buried in London?
My recommendation would be the kever of Rav Eliezer Gordon, renowned Rav and first Rosh Yeshiva in Telz, Lithuania, who died in London in 1910. He is buried in the Edmonton Federation Cemetery. The most “popular” kever destination is that of the famous Shotzer Rebbe, Rabbi Shulem Moshkovitz
Finally, I would suggest that, particularly in view of your family's connection to him, you visit the tziyun of the Sassover Rebbe, Rabbi Simcha Rubin, who died in 2003 and is also interred in Enfield.
Besurot tovot [hope to share good news in the future]
PS Although he is not buried in London and was not a gadol ba-Torah (leading Torah personality), a great Jew with enormous merits is certainly Sir Moses Montefiore, buried in a private mausoleum, next to his wife Judith, who predeceased him, on the grounds of the Montefiore Synagogue in Ramsgate (the destination of a Kesher trip a few years ago).