Judging the water levels in UK rivers is never easy.  There are various gauges that can be viewed over the internet on contacted by phone to check water levels.  For phone numbers and websites ...

Environment Agency Rivercall

The Environment Agency operate a series of premium rate phone numbers that give water level information on many rivers.  To see the full list click here.  However,  unless you can interpret the gauge levels given the telephone service can be rather uninformative. 

For a site that gives interpretation as well as the river levels visit http://rainchasers.com/