But, rain metal on what. Virtually all of a flight is not over any densely populated area. You know, that's why they call it fly over country? And flight schedules are notorious for being late by wildly varying amounts of time (never heard of one being early yet).
I think the first planes would have blown up as they circled New York and at the same time all the others would have been triggered as well.
Get a look at CNNi and see inside Heathrow...man, oh, man.