Of course.

BA like most arilines will release fares that involve lots of other airlines.

SCL-LON can be allowed as SCL-GRU-LHR or SCL-MIA-LHR with the first sector on LA or AA.

There are lots of domestic flights in the USA also which do not carry BA codes (many ex-USAir routes).

If the journey is not a BA published fare or if there is a mixture of AA and BA fares where BA is a minority, then it might not be permitted to issue 125 tickets.