Yes, African-Americans were always at the bottom, but there was no monolithic “whiteness” in the mosaic anywhere. Remember: when I was at Harvard in the 1980s, when an Italian-American Catholic married an Irish-American Catholic in Boston, it was a “mixed marriage”, and people would shrug their shoulders and say it was unlikely to last.
To an extent the same was true in the UK. Don’t forget, Catholics couldn’t build churches until the 1840s. The Navy was very much a Protestant citadel until well into the 20th century. Glasgow doesn’t exactly have an entirely harmonious mix between Paddie Papist and Prot today. The idea of a unified whiteness just wasn’t the historical truth at all. And given the mixed marriage – or at least mixed child parentage – rate in London today I expect those racial boundaries to decline rather faster than those earlier ones.