The online booking tool will not help in this instance. There is an advance booking requirement of 8 days for it, see here:
http://www.oneworld.com/ow/air-trave...inerary-online
Probably the best bet is to call BA now (morning UK time) and see if they can help. Their UK numbers are here:
http://www.britishairways.com/travel...out_Country=GB