Germany. UK and France has a small alliance prior to the early months of WWI, but not nearly as strong as other international alliances of the time. UK was, however, closely allied with Belgium and when they were invaded by the Germans, on their way to France, UK firmly found themselves allied with France. France welcomed it as they were in desperate need of an ally on the western front which they were losing.

After the war, they shared in victory and common hatred of Germany, which was only further solidified in WWII.

TLDR; France hated the Germans more than the British and both found an alliance of convenience that’s lasted over a century.