If her name is Anna Johnson, as you said in the OP, I'm afraid I would disagree with Rover and lauralie2 - which is rare for me as I usually concur fully with Rover_KE especially.
I believe that if a woman chooses to identify with a maiden name after her marriage, or retain a previous married name (perhaps to keep the same surname as her children) then that is her right and to ignore her desire to do so is the height of rudeness.
If a woman self identifies as "Mrs Anna Johnson" then that is exactly how she should be addressed, no matter what her husband's name happens to be. I would hope (and indeed believe) that this is an increasingly common point of view in the UK.
Therefore, I would suggest that the couple be addressed as "Mr White and Mrs Johnson". I do agree with Rover that the inclusion of first names or initials is optional.
I won't argue with him that the "formal" method, when both partners share a surname, is to use only the gentleman's name or initial to identify both partners, however you should be aware that a great many women - and men - nowadays will see this as offensive and sexist; it might be best avoided on those grounds. When I see it done, I try not to take offense, but it still annoys me!
Using only the man's initial also leads to issues with couples that are not MF but MM or FF and that is another reason to consider dropping it. It is my fervent hope that it will become an obsolete custom.
