Perhaps there was good reason for the Brits to leave their terror level at Critical, as in an attack is imminent.
Terrorists also in America...Italy...Britain - widespread terror attack planned? All on the same day. Iran?
I think that sometimes they keep the level high because people are more vigilant and more likely to report suspicions. THAT is more likely to help catch the perps. I don't think that the authorities are confidant we're out of danger yet.