Why is a 'w' called a 'double U' and not a 'double V?

Ah, children, children. The answer is obvious if you look at old printings. Way back when, the letter was always written with two connected "U" shapes, hence the "double you" phonetics. Modern typesetters, then typewritters (and thence computers) changed the appearance of the letter. Why? Originally, probably to save costs/effort in making the mobil type used by the typesetters.

hope that helps... ;-)