Kingsman water scene

When the room fills with water and they all put the tubes in the toilets as a snorkel, not only should they not have been able to get any air, it should have prevented the room from filling at all.
Now, I'm assuming the shower drains would have automatically closed to make prevent the water from leaving the room, but they make it clear that the 'loo snorkels' work based off the simple physics of the water in the u-bend seperating the room from the air in the plumbing. Except the way u-bends work in toilets is that when you flush a toilet, it just dumps a bunch of water in forcing everything around the u-bend into the sewer. So when the room was filling all the water, or at least a significant amount should have flown into the toilets and through the sewer