Back in the day that stairway off the main concourse, opposite platforms 8/9 gates approximately, was for some of us the preferred (and quickest) way down to the staff canteen that DelW mentions, after arriving on the blocks. The entrance to the canteen being another set of stairs down, which were in the wall (now blanked off) directly opposite the bottom of the first flight of steps from the main concourse, if that makes sense. Another often used facility sadly missed when it closed. Quite a few of the guys who worked at Waterloo would sit there on their breaks playing rounds of Domino's ISTR....happy days!
Use to be many years ago. Opposite platforms 3/4 gates, underground, entrance in the main building at concourse level and down a flight of steps. But they were closed and given over to a pub/wine bar many, many years ago. Can't now remember the name though...