snaga
The reason why terrorism is introduced into this is because it is the relevent subject. A bomb threat comes under that general heading, so it's whats being dealt with.
I can't give you a precise moment of passing control, because it depends on the specific circumstances. In general though, while the plane needs controlling in some way, I would not have interffered in that.
The question needs addressing to people who probably don't read this though. Try ACPO or SRG.