While I'm hearing bits and pieces on the news now that Pakistani authorities might have tipped off the Brits about a suspected plot, I have another theory.
Some of the early reports I've seen on TV and have read online were saying that the suspects were very close to executing their plot, with multiple suspects aboard each flight. It's also been reported that the airplanes were to have been bombed nearly simultaneously.
This can only mean that they'd already booked tickets, and the departure date was nearing. This leads me to believe that the British authorities, as much as we criticize them for political correctness, saw a red flag when a lot of Mohammeds and Ahmeds were preparing to travel to the US on the same day.
So for now I'll say "Good job, mates!".