Made it to the sterile area before boarding the Eurostar back to Europe!!!
I have my British passport and French ID with me. Interestingly the automatic gates don't work, probably because I didn't apply for any visa or anything since I have citizenship, so since the gate only sees my passport and not my ID, it has no way of knowing that I can legally travel. The border control looked at my passport and ID, and gave them back without stamping anything, which feels a bit freaky.
The French border cop smiled at me and said "merci!" when I handed a French ID