Why is 'w' pronounced 'double u' rather than 'double v'?

English uses the Latin alphabet of the Romans. However, this had no letter suitable for representing the speech sound /w/ which was used in Old English, though phonetically the sound represented by /v/ was quite close. In the 7th century scribes wrote uu for /w/; later they used the runic symbol known as wynn. European scribes had continued to write uu, and this usage returned to England with the Norman Conquest in 1066. Early printers sometimes used vv for lack of a w in their type. The name double-u recalls the former identity of u and v, which you can also see in a number of words with a related origin, for example flour/flower, guard/ward, or suede/Swede.

(Based on the Oxford Companion to the English Language)