What is the longest river in Britain?

The River Severn is recognized as the longest river in Britain, flowing for approximately 220 miles (354 kilometers) from its source in the Cambrian Mountains of Wales to its mouth at the Severn Estuary. Its significant length and the geographic trajectory give it this distinction.

In contrast, the River Thames, while being one of the most famous rivers in the UK and flowing through London, measures around 215 miles (346 kilometers), making it shorter than the Severn. The River Trent and River Ouse are also notable rivers but do not match the length of the Severn. Thus, when considering the longest river, the River Severn stands out as the correct answer due to its overall length and prominence in the landscape of Britain.