Specifically, Le Drian didn't say "veto"; his exact quote:
« Dans l'état actuel des choses, c'est non » (Given the present situation, it's no)

As tempting it is to present the A50 extension debate as a France vs. UK match, fact is: the EU Council can only grant one  by unanimous decision. In theory, every country of the EU27 can then block the extension, from the largest (Germany, pop. 85 millions) to the smallest (Malta, pop. 370,000). In practice, the Council always did reach a consensus, after much heated discussion for sure, but a consensus nonetheless.