Do you really need an UK address?
It could be that Iberia has really no office in the UK, because BA handles everything from an operations point of view.
I think in Germany it is like that:
According to EC261/2004 and relevant case law, a passenger may sue the airline at the port of arrival or destination.
If IB flies MAD-FRA -> The pax can sue IB in Frankfurt local court; the court will deliver to court paper to IB headquarters in Madrid.
Is that different in the UK court system?