Thanks percysmith. I wasn't thinking of changing to an MCA award at this stage since it would need to be another other airline besides BA or CX to do that.

I'm wondering if I can book say EZE-LHR as a one way and then call up and ask to have LHR-ZRH added to the same ticket so going through LHR becomes an EZE-ZRH ticket with an LHR connection a few hours later so it is still a one way ticket. If I wait until the next day I would be able to book EZE-ZRH as a one way redemption with both flights but I have to hope that there are still redemption seats for the EZE-LHR segment available 24 hours later.