I have noticed that F availability has been very elusive for a year or so. It exists but I see it come up in batches at very random intervals and it can disappear just like that. I think it is by design, probably in large part because of reduced F cabins and also because it can be priced well in a sale and therefore BA can largely fill the cabin by revenue paying passengers. In any case the huge difference in taxes, fees, (whatever you want to call it) between F and J has made it less appealing to me.