Tries from Ruth Laybourn, former rugby league player Watmore and Fiona Davidson carried England to their victory, with Amber Reed claiming a conversion and two penalties.
France had led 10-3 inside the opening 10 minutes after an exchange of penalties and a converted try from Jessy Tremouliere, with Aurelie Bailon adding the extras.
Reed narrowed the lead with her second penalty in the 18th minute, and England were handed an advantage when Elodie Guiglion was sin-binned soon afterwards.
England rallied with the extra woman and edged in front when wing Laybourn made the most of fine play from Danielle Waterman by sprinting clear to touch down.
It remained a one-point lead at the interval, but Worcester's Watmore then came off the bench and burst through at pace to reach the try-line and lift England to a 16-10 cushion, soon extended by two points when Reed converted.
England lost Heather Fisher to the sin bin and France bit into the home side's lead with a penalty from Bailon.
However England, back to their full complement, had the final say as replacement scrum-half Davidson darted over for the clinching third try.
England's next assignment is their three-Test series against world champions New Zealand, beginning in Esher on November 23.
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