There is no guarantee of F (Z) award availability on any flight, unlike CW where they guarantee 4 seats in each cabin. Typically the best time to check is at the end of a sale or if there is an equipment change to an aircraft with a larger F cabin (e.g. to A380).

The loss of the 747s and reconfiguration of the 777s that still have F to 8 seats only has made the BA F proposition much more exclusive and less easily available for reward redemption.