Problem is though, she wants to book a ticket and upgrade it all at the same time.

My understanding is that for her to do this, she has to login with her EC number first (to give access to the Book & upgrade functionality which verifies she has mileage available).

Then, when it gets to the stage of making the actual booking, the final screen where the CC details can be input, the (Spanish) address details are pre-populated from her EC registration.

There seems to be no way to enter the cardholders address as being in the UK.

Are you saying that they don't check the address against the card and we can just enter the card number etc and it should work?

She can't change her EC registered address to the UK, it was only changed recently to Spain, so we have to wait 6 months before making another change. Apparently.