The only official thing to show her married name is to change her passport into her married name, then to use the new name to other documents like bank account, etc. There's no rule that wives should use her husband's name, not in the UK anyway. She can continue using her maiden name even if she's already married. It's just what most people have been accustomed to, to have the wife use her husband's name, but it's not always necessary. I think in Spain, it's the husbands who use their wives names.