The past
The ‘w’ sound closely resembles the consonantal sound of the Roman letter now represented by ‘u’ or ‘v’, and it was originally written ‘uu’. In Old English the runic letter ‘wynn’, which resembles our letter P, was more common, but in Middle English it was written ‘vv’ (by that time ‘u’ was established as a vowel and ‘v’ as a consonant), which became ‘w’ in ligature form. The English pronunciation, ‘double-u’, is based on the ‘uu’ form, despite the fact the ‘vv’ ligature is much more frequent in English handwriting. In contrast, the French pronunciation, ‘double-v’, is based on the ‘vv’ form, despite the fact that the ‘uu’ ligature is much more frequent in French handwriting.
The present
In colloquial English the ‘l’ sound is usually dropped (‘duba-yu’), and frequently the second syllable is dropped entirely, leading to the British English ‘dub-yu’ and American English ‘dub-ya’. This pronunciation is particularly common in the acronym ‘www’, which occurs in most web address.
The future
The frequency of the acronym ‘www’ has lead to many proposals to change its pronunciation to save time. Although some proposals suggest a new pronunciation for ‘www’ as a whole (for example, ‘six-yu’), most suggest changing the pronunciation of the letter ‘w’. These include ‘dub’ as an even more abbreviated form, and ‘way’ as a form closer to the consonant-vowel pronunciation of most English letters (although ‘wee’ would be more logical, its sense meaning ‘urine’ makes it an unpopular choice). It remains to be seen whether any of these rival pronunciations will gain in popularity.