That raises the possibility that the so-called Real IRA may have worked with a group inside London's fairly large Muslim community to pull it off. I wouldn't be surprised at all if the Real IRA did this to exactly match the standard al-Qaeda modus operandi so they can't finger the Irish terrorists behind this.
Except that they have never exhibited the level of coordination required for an attack of this nature.
They may have helped, however, with intel or mules, in which case-God help them.