This is just breaking, link from sky news and the guardian:
http://www.guardian.co.uk/...
http://news.sky.com/...
Not much information is available in either story. Both of them say this is a long term operation. The Sky page says that the explosives would have been detonated over UK and US cities, and the Guardian says that the attackers were plotting to bomb flights from the UK to the US.
It looks like UK has the same color coded alert system that the US does, did not know about that in the past. Anyway, their alerts are raised to critical, so beware any travelers headed in that direction. The police have arrested 20 people,
from the Sky News description it sounds like they are mostly young men of Pakistani descent who were born in the UK.