There's no guarantee of F seats unlike Club. In general F seats are also released sporadically as Revenue Management decide they may have spare capacity and this tends to be much later in the day than the t-355 days when the 2 CW seats are released.

LAX is a route that BA regularly fill with revenue F passengers so is renowned as a tricky redemption in F. San Diego may well be an easier option. I would lock in seats in CW as soon as they are released and then check regularly for any F seats which you could call up and book for a £35pp fee. You can sign up for notifications via BARedemptionfinder. Good luck!

As an aside, if you really want F then Boston & New York are much more reliable options.