Some thoughts:
Is a camera (or other device capable of recording images) a prohibited item under the Prevention of Terrorism Act ? Not specifically as far as I am aware.
I understand that the Airport Authority can prohibit photography under its bylaws. You are on private property and the property owner can stipulate whatever restrictions it likes (providing they are legal) on your use of the property. Prohibiting photography is not the same as not allowing anyone to have a camera in their possession. Similarly, it's illegal to smoke when in the Restricted Zone but not illegal to have cigarettes and matches in your possession. Passengers access the Restricted Zone routinely and there would rightly be an uproar if the security bods confiscated all cameras. You will have noticed that at the entrance to passenger search areas, there are signs which specifically prohibit photography, not the possession of cameras. I think this restriction is in place by virtue of the Prevention of Terrorism Act. As you leave the search area there are no signs which lift the restriction on photography but I believe that Terrorism Act no longer restricts photography. However, the Airport Bylaws will still apply.
I believe the original reason for having such restrictions in place were to do with copyright. If you were for example to photograph a terminal building from private property and have the photograph published comercially without having the owners permission, you could be in breach of copyright laws. As ever, the security stasi are re-interpreting the bylaws to go beyond the original remit of "ensuring security". Most of the photography magazines have been running articles over the last year or so covering "Photography and The Law". Most concluded that "it's a grey area"..........