We never know, when we hear of various terror arrests, what havoc might have been wrought if the terrorists had remained at large. From [1] itv.com, with thanks to the Constantinopolitan Irredentist:

Britain's security services thwarted a September 11-style attack on targets including Canary Wharf and Heathrow Airport, according to reports.

The plot is said to have involved pilots being trained to fly into target buildings including London's famous financial centre and the world's busiest airport.

It is one of four or five al-Qaeda planned attacks, since 9/11, that have come to nothing, after the authorities intervened, reports claim.