Britain is a far less racially mixed country than the US is. Only about 2% of Britons are black as opposed to about 13% of Americans. That is, Britain has proportionally about 1/6th as many black people compared to the US.
Any gathering in the UK is therefore likely to appear to American eyes as less diverse than they are used to seeing.
As somebody who was there - look with 1600 guests, you'd expect about 32 black people in the Abbey - that is 2% of 1600. I would actually say that number was easily exceeded, easily reached from what I saw.
Wonder how many whites were at Obama’s wedding..
The main percentage of African-Americans are concentrated in the larger cities. In the Midwest where I live, there are large areas that have minuscule black populations. In the nineteen-sixties, my hometown of La Crosse, Wisconsin had exactly one black family living there while I was growing up out of a total population of 50K. Today, La Crosee has about five hundred black people constituting one percent of the city's population. Many towns and cities in the Midwest are like La Crosse. You can look for a long time in those places without seeing a black person.