In miles, what's the length of Britain's longest river, the River Severn?

The River Severn is recognized as the longest river in Britain, and its length is commonly accepted to be approximately 220 miles. This measurement takes into account the river's full course from its source in the Welsh mountains to its mouth at the Bristol estuary. The significance of the River Severn lies not only in its length but also in its ecological and cultural impact on the regions it flows through.

Choosing 220 miles accurately reflects this well-documented fact, which is supported by geographical studies of the river. In contrast, the other listed lengths do not align with the established documentation of the River Severn's total length, making them incorrect for this question.