If you look back in this thread,
@marni1971 indicated that Mary Poppins was an option, but at the time of specific posts he didn't think it was the lead option.
@WDW1974 said no Mary Poppins. Jim Hill on the other hand made a single suggestion and that was that the attraction WOULD be based on Mary Poppins.
Here are the relevant posts from my
rumor tracker:
August 17, 2017:
WDWMagic.com user marni1971 is reporting that the attraction being added to the United Kingdom Pavilion would be a ride.
July 26, 2017:
WDWMagic.com user WDW1974 is reporting that no Mary Poppins is currently in development for Walt Disney World.
July 20, 2017: On the
latest episode of The Unofficial Guide's Disney Dish podcast, Jim Hill and Len Testa discussed that the new ride coming to the United Kingdom Pavilion is expected to be a carousel themed around Mary Poppins. The ride is unlikely to be announced before Mary Poppins Returns debuts in theaters in Christmas 2018.
April 17, 2017:
WDWMagic.com user marni1971 is reporting that one theme being considered for the UK Pavilion addition is Mary Poppins.