Ive signed itntwice, once as a young bobby and once as a skipper. I could easily have declined, but I wanted to be part of the operations that necessitated it and they'd have just turfed me off if Id been defiant over that, no matter how hollow it is as the act does not require a signature in order to be valid.
Even without signing, the Act applies. In the context of official secrets the signing is irrelevant.
Lost in the midst of time and jokey memes is the idea that the signing makes obeying the act part of the contract of employment and breaching it a potentially dismissable offence. Without signing there is an Employment Law minefield of "no one told me, so you can't automatically sack me".
Now Positive Vetting (Been done twice*) is a very different Kettle of Very Smelly fish. There are very clear and direct implications in signing the declarations that are part of that.
*Granted a very long time ago - once as a Student before working a summer @Aldermaston and once during my time in the Nuclear Industry some time later @Sellafield or CEGB HQ, can't remember which.