According to Geoffrey of Monmouth
and other medieval British "historians," London
had been called "Trinovantum" because it was "New Troy
." They believed that Britain
was settled by Brutus
, son of Aeneas
, and so that the Britains
. (In an interesting aside, the Irish
believed they were descended of the Greeks. Sound like a familiar fight?)
However, as NatchLucid has pointed out, "Well, the dictionary gives "Trinobantes" as the name of a people in east Britain. Trinovantum as their city does make sense, b to v consonantal shift, but novus doesn't decline to make novantum. Novem, nine, would be closer, but that doesn't decline at all. I think it's far more likely that Trinovantum might be Romanization of a native name, and splitting to translate it as either New Rome or New Troy seems iffy to me."
So, the name is likely a medieval misunderstanding of a local tribal name, combined with this weird desire to be a part of Classical culture.
In legend, Trinovantum becomes Caer Lludd/Carlud, then changed to Lundein, and then to London. This is false, however, as it was the Romans who named the city Londinium. Why, I dont' know.
NatchLucid: perhaps the Britons where trying to appear more Roman, and so claimed a common heritage?