Piltdown Man,
It comes as a direction from the secretary of state and as such forms part of the National Aviation Security Program - I am not a lawyer but in the past I was pointed to
Aviation security act 1982 PartII Para 14. This empowers the secretary of state to issue directions on operators, airports etc and creates an offence by not complying with those directions. First reading points to mainly measures employing searches but as you go down through part two you will see that it does not confine the power to that.
As said, I am no lawyer but the offence, as I read it, is failing to comply with the direction.