Sorry - I didn't explain it well. The flight was the last flight of the day LHR-DFW, so we couldn't travel that day. BA then rebooked us onto a flight in J the next day (Dec 22nd 2018). There was no F availability at all that day. I don't know how far out they would have given us F - the app at the time only offered J, and I phoned and they told me J was all they had (and of course it was getting very close to Christmas).

I wrote letters at the time asking for 75% back under Mennens, which valuing the 2 for 1 voucher as equal to the fare paid for the other passenger would have been a total of 150k Avios + £240 by my calculation, but their position was that because we didn't push them for a travel on a future date in F (which would have been at least 2 days later) we were due nothing.