Having just had a very unpleasant back and forth with BA customer service, it’s clear they are under instructions to initially deny claims as much as possible.

As others have said, submit what evidence you have (including an initial eticket issued prior to any changes, if that’s the case), indicate that BA offer no CE fares without baggage, and if they don’t pay tell them you want the matter referred to CEDR then file with them.

A ticketing error isn’t your error, try as they may to not pay out.