Someone asked me this in one of my classes today.
The accent they mentioned is the Transatlantic or Mid-Atlantic accent, very common in movies from the 1930s and 1940s. It was also common among American heiresses married into aristocratic British families. I sometimes have a bit of an in-between accent myself. My South African accent has become milder over the years of not being surrounded by South Africans, and I also have to have a neutral accent when I teach, to prevent all my students from learning to speak English with South African accents.
