When is France's next traffic light review? (Covid travel)

Unlike the British government, the French government has not publicized any schedule for reviews. The official reference for the list is the arrêté du 7 juin 2021 identifiant les zones de circulation de l'infection du virus SARS-CoV-2. Legifrance lets you peruse all the different versions of the list and you will be able to see that's it's been modified almost weekly during June and July but not always on the same day of the week, with sometimes as little as 5 days or as many as 15 days between two versions.

There hasn't been any update in August and, given the general rhythm of activity within French government circles, it's entirely possible there won't be any before the end of the month. It's also late to hope for any significant effect on the summer tourism season so the pressure to relax restrictions is lower than it was earlier in the year (but additional restrictions could easily be added if the situation deteriorates).

All that to say that I wouldn't hold my breath for a quick change (and that's based solely on the political process, without even getting into any kind of speculation about the course of the pandemic in the UK).