Actually a 50 minute phone call to Avianca got us booked and upgraded with their LifeMiles (which happen to be on 2-for-1) Also, we are flying BA to Mexico City and then from there to Bogota, so can't book with Iberia. Although now I get to keep the Avios for next companion voucher

In summary - Iberia should really retract their information that you can book Avianca with their points, as I have found no evidence that anyone has ever managed to do so. Avianca on the other hand, really helpful. Columbia here I come!