I can't guarantee that they were never open to the public, but I can say that the waterloo steam tunnels were definitely not designed to be accessible. The tunnel from V1 to main campus is narrow and has a ladder about halfway. Also the steam pipes and many building controls seem to be out in the open and unprotected.
I'm not sure how this photographer got down, but about a decade ago there was a way to sneak in through an access panel.