Hello, thanks for replying!
I've found the 1939 Register, with her living with my great great grandfather, Samuel Darbon and their children in Acton. This document reveals her birthdate but not birthplace nor her parents. Annoyingly, no 1911 or 1921 census has Rebecca with a father called Victor Schmidtt/Smith (Something my great-grandad was very insistent about). There were a few contenders, but none with Victor/their birthplace being Poland, Russia.
In terms of a marriage certificate, I have the civil marriage in 1924 between herself and Samuel, but this didn't reveal anything about her parentage.