Lennox and Besser, both 57, met in 2009.
Britain's Mail Online reported actor Colin Firth and comedian Ruby Wax were among the 150 guests who attended the wedding ceremony Saturday on a private boat on the Thames River. Lennox's daughters -- Lola, 22, and Tali, 19 -- served as bridesmaids, the newspaper said.
Besser founded mothers2mothers, a private organization based in Africa and dedicated to reducing mother-to-child HIV transmission, People.com said.
Lennox, an activist for HIV charities, was previously married to Radha Raman and Uri Fruchtmann, the father of her two daughters.