I flew with Iberia express as a BAEC Bronze, the booking was made via Amex, but was IB Express with an Air Nostrum connection in MAD on the outbound, and a BA return on the way back. I think it was made simpler as I was able to MMB on BA.COM as one of the flights was BA. Avios and TP credited quickly and I had my BAEC number on all the boarding passes.

Only snag was the BA lounge at Gatwick not being able to scan my business class IB express boarding pass for entry into the lounge, after much debate and checking they let me in. Now I'm Silver this situation would not arise, but other than that it was all fine, apart from IB express's poor Business class product and couldn't care less crew who hid in the galley curtain drawn after the meal service!