Plus if Bond were to go to London, that would make the attraction/area lineup for London: Diagon Alley + Gringotts, MoM + its respective attraction, aaaaand Bond. It would stick out like a sore thumb.
Plus, this is all speculation, but won't they need to re-brand "The Wizarding World of Harry Potter - Diagon Alley" to "The Wizarding World of Harry Potter - London" once MoM opens? Marketing it as Diagon Alley in USF would completely leave out MoM, but having "The Wizarding World of Harry Potter - Diagon Alley" and "The Wizarding World of Harry Potter - Ministry of Magic" in one park just seems convoluted.