As the flight was departing from the UK, and IB is not part of any Alternative Dispute Resolution body, you can complaint through the CAA when Iberia ignore / dismiss your claim. Normally it takes 2 weeks to get a reply and in my experience IB will pay.
What to do if your claim is rejected | UK Civil Aviation Authority
https://caaportal.icasework.com/serv...eKtw%3D%3D~%21