Jamie Foreman who plays Alf White, head of the White family street gang, is the son of real-life convicted London gangster Freddie Foreman who was convicted of handling the proceeds of an armed robbery. He was also convicted of disposing of the body of Jack "the Hat" McVitie who was murdered by Reggie Kray.
Jack Comer was born Jacob Colmore in Mile End, in the East End of London, England on 12th April 1912 and died 12th March 1996.
Frankie Fraser was born in Cornwall Road, Waterloo, London, England on 13th December 1923 and died 26th November 2014.
Billy Hill was born William Charles Hill in St Pancras, London, England on 13th December 1911 and died 1st January 1984.
The song sung and covered within Once Upon a Time in London (2019) of "We'll Meet Again" (originally titled "We'll Be Together Again") was a [British] World War 2 song sung by Vera Lynn and renamed after the film, starring Vera Lynn, We'll Meet Again (1943).