OP - just FYI - I'm on a 4-class B77W on today's LHR>BLR sector and F was blocked out until 2 days before flying. I looked on the app seat map (and BA.com) and they weren't available for selection, so I assumed they came off block, but weren't for use. Upon boarding, the entire F cabin is full with pax, so I would assume these seats are only available for CW pax who call up and request specific seats. I would suggest subscribing to EF to see what aircraft you are on (not a guarantee obviously) and putting in a seat alert notification so if the F cabin becomes available, you can call and grab some seats.

Pilot37