I'm not sure why Ms. Brown didn't get Nancy, although I think the people making the film wanted to soften her up somewhat from how she played her. Among those considered were Julie Andrews, who was top of the list at that time for any female musical role, and Shirley Bassey who had a hit recording of "As Long As He Needs Me". I read that the studio wasn't comfortable with multi-racial casting with Ms. Bassey. Andrews, who had mastered Cockney for her Eliza in "My Fair Lady", eventually wasn't available.
I love Georgia Brown on the OCR, and it's a treat seeing her and Davy Jones of the Monkees on the clip on "Ed Sullivan" which was on the night the Beatles made their U.S. tv debut. I think Shani Wallis was excellent in the film. I believe she got fine reviews for her singing and acting, the biggest criticism being she looked too sunny and pretty for the role. I think Ms. Wallis had gotten a big national opportunity on "The Ed Sullivan Show" singing from her then-running Broadway show "A Time For Singing" which was seen by at least someone involved with the film casting of "Oliver!". I was rather surprised she never really had a big opportunity again.