I am not a lawyer, and everything I know about UK marriage law on bigamy for the period is what I learned watching a genealogy webinar about bigamy in colonial Australia. So with those red flashing caveats:

If Francis Moulton had revealed himself after the wedding, Hattie would not be guilty of any criminal offense; the bigamy laws were quite clear that after a long enough separation or if the remarrying spouse had reason to believe they were widowed, they didn't commit a crime by remarrying. I don't know, however, whether the revelation of the first marriage automatically makes the second marriage invalid -- Sayers stories suggest yes, but there were certainly some cases in the aforementioned webinar where the person continued to live as married with their second spouse even after their first spouse turned up.

Because he revealed himself before the ceremony was completed, the next question is whether Hattie's marriage to Francis was valid under English law. It's clear from the text that the marriage wasn't consummated until after Hattie and Francis were reunited, and if Hattie had wanted to stay with Lord St. Simon, her lawyer probably could have made the argument that the previous marriage should be annulled since it was unconsummated. In the circumstances in-story, no idea.