Scotland Yard believes that the "main players" have all been detained. Most international flights have been either delayed or cancelled anyway.
And they can't say they foiled the whole thing b/c they just can't know that right now. Security has been jacked up everywhere (national Guard are now in Boston).
That doesn't mean everyone should go into "oh shit!" panic mode, and bring everything involving planes and travel to a halt. That's, uh, kind of what these guys were going for.