No prior generation of people, in the 250 or so prior generations throughout recorded history, ever got themselves into a position to Exterminate their own people, and went forward to do just that.
The English and French did a pretty good job during the hundred years war in Europe. The English King claimed the provinces were his, and went ahead with killing off enormous numbers of them.
The Cambodians did a pretty good job under Pol Pot.
The French killed off a large portion of their population during the reign of terror after the French revolution.
I also thought of the Cambodians, but the goal of that government, at least as stated, was NOT to exterminate their own race and replace it with other races, but rather change the country to some sick, agrarian, culture.
As to the UK and France - yes, genocides certainly happen, even self-induced, but I still haven’t seen any examples of a culture DEMANDING mass migration of foreigners who OPENLY plan to replace and likely exterminate their existing culture/race (in this case, Whites)...until the Baby Boomer generation.