Hmm.. So I booked a (non-economy basic) non refundable flight using the FFC + some very small fare difference. Waited 24 hours, and then cancelled it. I received a new FFC, still under the original PNR, with the original "travel by" date (end of this week).
Somehow it seems like the trick to extend FFCs doesn't work anymore? Or perhaps it's again because my original booking was an international segment (and in GBP not USD).

I can't fly this week. I guess if this doesn't work, I should just let this FFC expire worthless? It's worth ~$1,000 so not great.
Or is there a chance I could book a fully flex ticket and then instead of cancelling it, changing the date to sometime in the future (if they let me, despite the "begin travel by" date)?