The point, surely, is that this is a BA assistance line, not a oneworld one. The individual airlines may well have a massive amount of potential numbers to call throughout the world which could also be dependant on status level, on top of which there may well be website chat options. I wouldn’t remotely expect a BA line to have all of that information at their fingertips, so if I was told to contact another airline I’d expect to go to their website and find an appropriate method of getting hold of them myself.

BA will, I’m sure, transfer some more complex calls to codeshare partners - I’ve been put through to AA by the Gold line with a query no-one at BA could resolve and which needed a high level agent - but for simple things such as seat assignments that simply doesn’t make sense.