Aside from V and U sharing the same spelling, there are a number of alternations between them in etymologically related words inside Latin. For example:
These alternations involving V make it more plausible to suppose that Latin [v] developed from [w], because [w] and [u] are more similar and more likely to change into one another than [v] and [u].
The sound [u] is phonetically categorized as a "close back rounded vowel": "rounded" refers to the shape of the lips, which are brought together (there is a bit of variation between languages in the exact position of the lips). The sound [w] is pronounced mostly the same as [u], differing mainly in being non-syllabic. The sound [v] is further distinguished from [u] in several respects: 1) it is "labiodental", being pronounced with the lower lip against the upper teeth; 2) it has greater constriction than both [u] and [w], being categorized as a "fricative" consonant; 3) it lacks a "back" component.
These considerations don't tell us when [w] would have changed to [v]. So this isn't an argument for [w] being the value of V in "ancient Latin", just in some stage of the language that eventually developed to Latin. The use of [w] in distantly etymologically related English words supports the reconstruction of [w] in the common ancestor that English and Latin share, Proto-Indo-European.