I received a reply to my latest boarding pass photo email yesterday, I've been told to speak to Iberia and request they send me a 'Flight Confirmation Letter', and then forward that on to QFF.
For those in a similar situation, I've found on Ibera's website a 'proof request' form to obtain a proof of travel certificate: https://www.iberia.com/gb/customer-relations/#tab-2
I'll report back if that works, and of course if it's finally accepted by QFF.
