Well, France is and was as diverse as Britain. Many thanks to French colonialism and so forth. It’s the aged 50+ White racists that keep saying these period shows aren’t historically accurate unless everyone is White. Like there’s no historically-driven reason why a little Black boy couldn’t play Oliver Twist, for example. Call the Midwife is set in the 1950s / 1960s and in my all-time Top 10 shows ever made. Really worth watching.
If you want to know about the Windrush, I don’t mind sending you some links to look at. But in a nutshell, the Windrush Scandal was the Tory (Conservative) government deporting Caribbean migrants back to the Caribbean, many of whom had been here since 1948 (by then Home Secretary Amber Rudd), under a racist foreign policy devised by now PM Theresa May.
In regards to David Oyelowo, he’s one of the greatest actors alive and I recommend watching every bit of his filmography. The guy, along with filmmaker Amma Asante has done wonders for representation on screen in Britain. The conversation is changing and it’s got much farther to go.