Colleagues working on the ground at LHR had to get a special permission letter to take photographs airside, for personal use. Commercial use requires some sort of licence/permit with HAL...
I'm sure there is something in the airport by laws covering this too. Particularly sensitive areas are baggage make-up and baggage reclaim. Not particularly sure why, but I'm sure we've all spotted the signs whilst waiting at baggage reclaim which have the 'no photos' image on them.
