A BBC article from 2012 states that the River Severn is officially the longest river in the United Kingdom but that the River Thames might actually be longer. The discrepancy originates in a dispute over the source of the Thames.
The BBC notes that, as of 2012, the Ordnance Survey map shows Thames Head to be the official source of the Thames. There are, however, those who claim that the source of the river is at Seven Springs, which is close to Cheltenham. If that is considered the source of the Thames, it would be 9 miles longer than the Severn.