A couple of things:
Signing a document that refers to the Official Secrets Act is simply stating that you understand your responsibilities in terms of the act and this is done because you are expected to routinely come into contact with classified material.
Everyone in the UK is still covered by the act; you are just acknowledging that you really understand your responsibilities. I Have had to sign said pieces of paper more times than I care to remember over the past few decades.
When I was in the service (many many years ago), it was clear that items were routinely over-classified. Even the magazines that were delivered were stamped 'Restricted' - the lowest level but still a security classification.