Just heard CNN Intl define the critical threat level as an attack is imminent.
If they thought they had caught all the terrorists, they would not say that a threat was imminent. This is a CYA in case they did not get them all. I would not fly across the pond for a few days. Work be damned.
These are the British threat levels:
critical - an attack is expected imminently
severe - an attack is highly likely
substantial - an attack is a strong possibility
moderate - an attack is possible but not likely
low - an attack is unlikely