Role in the Story
While having sex with Draco, Ebony falls asleep. In her dream, she sees Serious and Lucian being shot by a black guy, who then escapes in a red car. She and Draco go and tell Vampire, and all three go to warn Dumblydore. When Dumblydore asks Ebony where Serious and Lucian are, she replies, "Longdon." Dumblydore then "went and called some people and did some stuff." Eventually Serious and Lucian are discovered and brought to Hogwarts on stretchers.
"Longdon" is actually a real English village located in the West Midlands. However, what Tara Gilesbie most likely meant was London, which is the capital of both England (where Tara thinks Hogwarts is located), and the United Kingdom. It is the most populated city within the UK, with a history going all the way back to the Roman Empire.
In the original canon, London plays an important part in the story, insofar as locations go. It was in London that the Ministry of Magic was located, as well as The Leaky Cauldron, Diagon Alley, St. Mungo's Hospital, and King's Cross Station (home to the famous Platform 9 ¾). It was also in London that Wool's Orphanage housed Tom Riddle before he was taken to Hogwarts.
The incident with Lucian and Serious in My Immortal seems to be taken from an incident in the books/film where Harry Potter has a vision of a character being attacked at the Ministry of Magic, which would have thus also been located in London. This seems to be the only connection Tara makes between London and the Harry Potter canon.