I had my conversion Mikvah with Masorti. I had Mikvah lady for the same Mikvah as you went. It was all organized by the Masorti Rabbi's. Maybe you should contact the New North London Synagogue. You should ask for Rabbi Jonathan Wittenberg or Rabbi Chaim Wener. They were part of the Beit Din and arranged everything else. The Mikvah lady was a nice elderly lady. I did not feel embarrassed with her as a witness. I am usually very embarrassed in front of strangers. I am surprised you did not get one.