I've never heard an FA say that a plane was cleared for take-off. Generally, the FAs make no announcements after the safety briefing until after take-off. The pilots usually make an announcement aimed at the FAs that take-off is imminent and that they should return to their seats. As an avid Ch. 9 listener I can assure the OP that this is some considerable time before the plane is cleared for take-off as usually the take-off clearance is followed immediately by engines revving. Indeed, at LHR, they are required to take off immediately they receive clearance, which will only be after they are on the active runway. The FAs will have been seated well before the plane is on the active runway.