I work for ICTS at Gatwick. I been told we are not allow to take photos and if we see anyone we have to inform them that they cannot take photos. As the company I work for is required to follow DSA requirements I always assume this was the reason. However someone from blackjack once mention to me that BAA does not allow people taking photos within the airport. This is also the same with video recording.
ICTS are so strict with the photo taking policy that if a staff member is caught with a camera phone, they will receive a written warning.