The bottom line is that, based on some recent posts, you CAN often use a CC not registered in that country. The payment still goes through without any problems as long as the name matches.

On a separate note, I have never been clear what this para from the MFU rules means: "MFU can only be applied to bookings which are made in the members Executive Club home country." Does this mean that your MFU flight has to start in Spain, or simply that you have to book through the Spanish website?

I am assuming that all of the household account members are registered in Spain? Otherwise you could do the booking from your UK account for your wife.