So much for genetics, which was your assertion.
Over time, high-IQ individuals leave the population, leaving the low-IQ individuals behind to marry others from the welfare class, reinforcing the low-IQ genetic aspects.
Stating a hypothesis is not a proof.
You seem to be operating off the straw-man that I'm saying that all white kids are inherently smarter than any black kids. I'm not. I'm saying that parents who have well-below-average IQs are more likely to produce below-average-IQ kids than above average kids.
Thus the kids of multi-generational white underclass of England are likely to be of lower IQ than any kids which Thomas Sowell would produce.