To produce return periods from river flow information is technically complex, expensive and will not always provide reliable results. Therefore, we currently have no intentions to publish return periods for recorded flows at each gauging station.
Experienced hydrologists can find high-flow data on the Centre of Ecology and Hydrology website for over 1000 sites (http://www.ceh.ac.uk/data/nrfa/peakflow_overview.html). You will also find advice and links to the Flood Estimation Handbook which sets out the methods needed to calculate return periods. http://www.ceh.ac.uk/feh2/fehintro.html.