CNN via ITN: Security alert on a London bus near King's Cross, somewhere on Gray's Inn Road (a very busy area). Traffic has been stopped, the nearby tube station has been evacuated, and "a lot of police around the area."
There are reports of smoke on the bus, police say some sort of fire is involved but no explosions; a newscaster reports, "People are walking around, trying to get to their destinations." Translated from the Queen's English, this roughly means, "We know nothing."
While you work on keeping your emotional terror levels away from vermillion, rest assured that most Londoners are probably rolling their eyes. These people have been to hell and back, so this sort of stuff must be slightly irritating.
Update: Scotland Yard is telling CNN that it's just an engine fire. False alarm, etc. Yes, you can be annoyed at the jumpiness, even if it is understandable.