As to "when" - The terrorist attack happened starting at 08:46 Eastern Daylight Savings (USA) Time zone on Tuesday, Sept. 11, 2001. USA Eastern Daylight Savings time zone is 4 hours after Greenwich mean time, so that means it would have been just after Noon, London time (if I did that right.) It was televised as a network news event. We had a TV in our "break room" so someone heard the news bulletin almost immediately and called us in.
I heard about the first plane hitting the north tower (of the World Trade Center's two towers) within a few minutes of it actually happening. The second plane hit the South tower shortly thereafter, and a plane hit the Pentagon. A fourth plane crash-landed in Pennsylvania and was thought to have been aimed at either the Capitol building or the White House in Washington, DC. (Which of those two was the target is not clear.) The passengers overpowered the highjackers but could not safely land the plane.
Many of us have already answered the "where" part of your question. My business day started at 07:00 so I was in my place of work, as described in an earlier post.