With the new BA seating policy it is impossible to book exit row seats more than 24 hours in advance, and even then you still have to pay an additional charge.
IMHO it appears that BA Exec Club do not know their own new T&Cs and also that you should never have been allowed to book these exit rows in the first place.
To be fair, I have never been able to pre-book emergency exit seats with any airline, but I am sure that there will now be hundreds of replies telling me I am wrong!