The Thames barrier's in place to stop the sea coming in, so we can't necessarily open it to let water out that's come down from upstream...
Here's how I understand it, but I could be wrong:
The Thames channel in London is big enough to hold the normal downstream water and the incoming tide, so the level of the embankments is more or less the same as that of the sea at high tide. However it's much higher than the sea level at low tide. So if you close the barrier for a bit at high tide the river will fill somewhat, but when the tide drops you regain enough gradient to continue draining the upstream water.
I'm just trying to justify not going to London today. I think that's reasonable given I have no meetings or anything special going on.