It seems odd if you can do it that way but the website does simply say:
"Atrium Bar: 12pm–1am, no reservation necessary."
http://www.aquashard.co.uk/
Why not give them a call and say that (for example) you'd like to meet a business colleague there for a drink but you're unsure as to how to enter the building without getting stopped at the tourist entrance?
020 3011 1256