America’s is segregated by race. You can’t mix with the other race or borrow something from them, or even sit on their table.
In Britain the races mix, there’s no strict segregation, if there is any, it’s more segregated by town and county and areas.
That was not true in any of the prisons I was in in Texas. There were four that I lived in over ten years and several others that I traveled through. I've heard about it being the case in some prisons, but to present it as a universal truth is wrong. The most strictly segregated prison system I've heard of is California's, but it's based on alliance rather than race alone. The AB-aligned whites are allied with Sureños (a Hispanic group) while blacks are allied with Norteños, and fraternization is fine within the alliance but not outside of it.