“On the basis of the interrogation of suspected Al Qaeda agents, Indian intelligence believes that Al Qaeda had infiltrated a suicide team into Britain to attack the Houses of Parliament. In addition to striking Westminster in an airborne attack, they also had plans to attack Tower Bridge on the river Thames, but the latter target was dropped in early September [2001]. The suicide team was to hijack a British Airways flight bound from London to Mumbai on September 9, 2001 (the original date scheduled for the Washington and New York attacks) but in an encrypted email received at the last moment from their Al Qaeda operational commander in America, the operation was postponed to September 11, 2001. The Al Qaeda cell in Britain chose to hijack a domestic London to Manchester flight, for which they assembled at Heathrow airport around 14.30 on September 11. The flight was scheduled for 17.00, but by then the US strikes had taken place and the authorities at Heathrow had quickly grounded all domestic and international flights.”
– Rohan Gunaratna, Inside al Qaeda, Pages 119-120