Great Britain isn't demographically and ethnically diverse? Have you ever seen any photographs and video footage of the citizens of modern British cities?
Where are you coming up with this crap?
To appear ethnically diverse, they have to count "White (other)" as a minority. Those are probably Belgian and French. Their next largest ethnic group, Indian, is only 1.8 percent of the population. Then it's Pakistani, "White Irish", and "mixed race."
At 92.2 percent white, they are as diverse as New Hampshire, without of course all the Swedes, Germans, Italians, and French, and only slightly more diverse than Vermont, which is 94 percent white.
By American standards, they aren't even a little bit diverse. We're 63 percent white. 20 percent of American children don't even speak English at home.
ETA: Strangely enough, Vermont, one of the few states with less diversity than the UK, also has a lower homicide rate than the UK, even though pretty much their only gun restriction is that you can't carry one in a courthouse. In one stat I saw, they were listed as having one arrest on firearms charges, so apparently someone did that.