Your problem may be that F is not a fare class that is eligible for the award of BA miles or tier points and so MMB is confused.
See:
http://www.britishairways.com/travel...d/public/en_us
From ba.com:
Code:
Eligible for Executive Club Tier Points4
Business class J, C, D, I
Economy (full fare) Y, B, H, T, K, M, P
Economy (discounted) A, G, L, N, O, Q, R, S, V, W, Z
4Discounted economy classes are eligible for 50% tier point accrual.