I'm quite sure there's more underground than we're allowed to know
about, but doubt it involves the LU system. There are just too many
people who would have found out about any such provision over the
years, 9999% of whom have never had to sign the OSA Act and are thus
free to speak.


I think you mis-understand what "signing the Official Secrets Act" means.
Signing it merely acknowledges the fact that the provisions of the OSA have
been brought to your attention. It does not impose any additional conditions
upon you. Everyone is bound by the OSA; signing it just gives you less
mitigation should you breach its provisions (not that there are many get
outs even if you were not aware of the provisions of the OSA).

Regards
Jeff