"Well, there you are, boy. Platform nine--platform ten. Your platform should be somewhere in the middle, but they don't seem to have built it yet, do they?"
     -- Vernon Dursley

London is...

• ...the location of at least some of the offices of the Ministry of Magic
• ...the location of the Museum of Quidditch (QA)
• ...where you'll find:
• Charing Cross Road, the location of The Leaky Cauldron (PA3), a small pub located between a book shop and a record store. Behind the Leaky Cauldron, in a dead-end alley containing nothing but a few dustbins, is the entrance to Diagon Alley. 
• Kings Cross Station, a typical railway station. Between Platform 9 and Platform 10 is a barrier and a ticket collector's stand, but Witches and Wizards can push right through into Platform Nine and Three Quarters. There, under a wrought-iron archway with the name of the platform on it, you will find the Hogwarts Express leaving on September 1 for Hogwarts and the start of term. The Express comes and goes periodically throughout the school year (SS6, 17; CS5, 18; PA5, 22; GF11, 37)
• a private hospital where Dudley had an operation to remove a pig's tail from his behind on September 1, 1991 (SS6).
• the Underground, where Hagrid got stuck in the ticket barrier and drew attention from the Muggles for knitting a huge yellow something or other, crammed into seat much too small for him (SS5).
• Paddington Station, where Harry and Hagrid had a hamburger for supper before Harry caught a train back to Privet Drive (SS5).