From a Time report on the investigation that led MI-5 and Scotland Yard to the terrorists plotting to blow up the transatlantic flights:
Britain's MI-5 intelligence service and Scotland Yard had been tracking the plot for several months, but only in the past two weeks had the plotters' planning begun to crystallize, senior U.S. officials tell TIME. In the two or three days before the arrests, the cell was going operational, and authorities were pressed into action. MI5 and Scotland Yard agents tracked the plotters from the ground, while a knowledgeable American official says U.S. intelligence provided London authorities with intercepts of the group's communications. . .
Seems like that NSA surveillance system has some good uses after all.