Can’t speak for the UK, but in the US, literally every terrorism prosecution that involved a foiled plot in the past ten years or so has involved the FBI convincing some helpless, directionless loser to do a bad thing and then arresting said loser and claiming victory against Al-Qaeda. The entire plot had been made up by the FBI.
That’s not to say the guys who agreed to said plots were blameless. That’s to say that I’ve seen zero evidence that the FBI has brought any actual, independently motivated terrorists to justice by foiling their plots.