As noted by stifle, david-a, and others, if you have set up a BAEC household account you can only use your avios to book for others members of the household.
From ba.com:
16.1.14. Subject to Clause 16.1.12, Rewards can only be issued by British Airways to members of the Household Account (and will not be issued to persons that are not members of such an account), as designated/nominated by a Household Account member at the time of issuance, after which Rewards may not be transferred or reissued to a different name except where in accordance with the Conditions of Use relating to Transfer Avios.
And from the Your Guide to Household Accounts thread:
Limitations of HHAs
If you set up a Household Account then you will NOT be able to redeem Avios for someone outside the HHA, unlike standard account holders.
http://www.flyertalk.com/forum/briti...-accounts.html