Oh memories! I worked in ec for quite a few years, the floor in ec1 drops (well used to!) to create a swimming pool, it was used every year for the boat show. Some of those “boiler rooms” you saw would have been the controls for the hydraulics to drop the pool top and the tanks to fill it up, there’s also service tunnels under ec2, containing buzz bars, all the electrics for the stands in the exhibitions were run up from there through ducting in the arena floor to power the stands. Those halls were always empty like that when no shows or exhibitions were in residence, every single show was built from scratch, then completely cleared at the end.