I finally got the rebooking page to load. Nothing available until 26th, and even on 26th, it's J only (thankfully I am in J...) but I would like to get back before 26th, since I'm leaving again on 27th!

Now onto searching for weird routings. Eurostar to Paris/Brussels and then flying is not an option since Eurostar is unavailable for booking until after Christmas. Ferry? Walking?