The flights targeted were allegedly on three US airlines: American, United and Continental. Heathrow Airport, the world's busiest international airport, was the departure point for the targeted flights. The explosive of choice was likely nitroglycerin in liquid form that would be charged to detonate by an electronic pulse. See here for how that could work. This is not an idea that takes great imagination today -- any Brit could try to pull off such a plan just by reading the opening chapters of this book (which is a highly entertaining one, by the way). Indeed, the whole concept of liquid explosive with separate detonator to be put together on plane after carrying the kit on with the carry-on luggage that Chris Brookmyre describes in that book is almost exactly the set-up that the UK bombers were allegedly going to use.
Then again, the concept of flying a plane into the White House or other US landmark appeared in Tom Clancy's Debt of Honor about seven years before 9-11-01 and the Feds never foresaw that possibility. So good on ya to the Brits for handling this.
UPDATE: The Brits sent what should be either (a) superfluous or (b) insulting -- a warning from PM Blair to President Bush to ensure that the ongoing operations to foil Islamofascist death plots remain CONFIDENTIAL.
For some US officials, Blair's reservations about US agencies' ability to keep secrets is an embarrassment [actually, it's the agencies' inability to keep the information from the press that's embarrassing -- TKM]. Blair's fears stem from a series of news stories that have exposed US counterterrorism operations and the potential for politically-motivated leaks.
Meanwhile, according to a source in the FBI, the directors of the CIA, FBI, NSA and other agencies have been briefed about the UK situation by White House officials. During the briefing, they were cautioned to prevent any breaches in the security of classified information.
Prosecuting leakers would help stop that nonsense.
UPDATE II: Kudos to Pres. Bush for cutting the [scatological euphemism] and speaking plainly.
President George W. Bush said on Thursday a foiled plot to blow up airplanes was a "stark reminder" that the United States was "at war with Islamic fascists."