Thanks all - stands to reason it is indeed some 'political' reason: either to do with shareholders, or with the terms of the existing JV (more likely?) - didn't realise that JV already existed, though seems pretty one-sided compared with the other JVs out there (BA/AA and the like...)
Exceptions notwithstanding (such as the old QR 777 from some EU destinations), it has to be something like this rather than there being a significant / lucrative contingent who are actually choosing to fly BA over QR in the premium cabins on this route...