Paul Kent  has kindly provided us with the link for the river level monitoring, which is:-

http://apps.environment-agency.gov.uk/river-and-sea-levels/default.aspx

Use it by selecting the area you want, followed by the monitoring station nearest to your swimming area or river activity spot.

The levels do not directly relate to flow rate, but it only takes a few times to work out what are safe levels for any monitoring station.