Alexandra Palace - one of the 'People's Palace's' from the Victorian era (the mostfamous one being the crystal Palace). Just after completion it burned down a week later, only to be painstakingly rebuilt on a new plan.
A vast complex that noone knew what to do with by the 20th Century -and to this day are still stumped (a part-time concert venue, exhibition hall and ice rink on only one section). It's just too massive.
Many of the ruined sections are slowly coming back into use, such as the theatre hall:
A big bone of contention is the art deco radio mast that got plonked at one end - wrecking the building but of high historic value as it was the first place that regular tv was ever broadcast from (yes the Brits invented TV).