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 were
Trojan. (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?