"The barrier has no individual trigger level for closure. Hydrological and meteorological data is fed into the control room every 20 minutes by telemetry. The closing process is guided by a mathematical matrix considering fluvial flow, tide and surge from this data. The end decision for closure lies with the Thames Barrier duty controller at the time."
I'm guessing, and Poly may well correct me, but there's a chap in a control room and he has lots of data at his disposal, he keeps an eye on it and if he feels there is a flood risk from a tidal surge then he closes the barrier, having a website listing probable closures would be like having a site predicting the weather.