From Time (HT: Drudge):
“…a knowledgeable American official says U.S. intelligence provided London authorities with intercepts of the group’s communications.”
UPDATE:
An e-mail:
Because specific individuals were already being tracked by the Brits, the wiretapping almost certainly proceeded by virtue of a court-ordered search warrant. The surveillance was not of the NSA data mining variety, but very likely similar to that used by previous administrations.