[HAL9000]

So why wouldn't the conspirators wait until then, when the airline security bar on international flights will inevitably be lowered?

I'll speculate that the plot was intended to be carried out on Sept. 11th - for obvious reasons - but the authorities prudently decided to move in now.

[Xenalyte]

FWIW, I heard this morning that the men weren't actually seated on the plane, but that seating was close, which would tell me that the attack was imminent.

Of course, there's no telling how correct early reports are.

[browardchad]

I'll speculate that the plot was intended to be carried out on Sept. 11th - for obvious reasons

My guess is sooner than that, or we wouldn't have this massive disruption of airline traffic today. We know first reports are usually misleading, but I get the feeling, based on mention that the plot was known for awhile, that perhaps the Brits thought 9/11 was the target, but some development proved that a wrong assumption.

OTOH, if this was a lucky catch by British intel, it would explain pushing the panic button today, since the plan's details would be sketchy.