The United Kingdom's longest river is the River Severn, which is 6 letters long. The River Severn is a river in the United Kingdom that rises at an altitude of 2, 001 feet on Plynlimon, Wales. It flows through Shropshire, Worcestershire and Gloucestershire in England, before discharging into the Bristol Channel. The Severn is the longest river in the UK, with a total length of 220 miles. It is also the second longest river in the British Isles, after the River Shannon in Ireland.