Are you booking one-way? If so there may not be any one-way fares.
If you are changing through the UK call centre, the UK POS would apply.
ExpertFlyer defaults to US POS, UK POS shows only J availability:
Code:
Flight Availability Search
Departing ORD on 03/29/14 12:00 AM for LHR
Flying BA
Point of Sale: UK
BA 294 0 ORD LHR 777 Unavailable F7 A7 J9 C0 DC RC IC W9 E9 T9 Y9 B9 H9 K9 M9 L9 V9 S9 N9 Q4 OC G9
03/29/14 6:05 PM 03/30/14 7:50 AM 85% / 21m