I very much doubt you would get the same result from France, given the very different historical attitudes towards black people in the two countries, as well as a higher number of black people whose stories are taught (due to the difference in attitudes).
Name a famous black person from pre-revolutionary America that more than half of British would know.
Clearly, it’s impossible to do so, given the story we’re commenting under. I assume sally Hemings or (probably less likely) crispin attucks would be the figures they’d be most likely to name.
The more relevant aspect of the colonizing point was that Britain colonized Kenya (as an example) in the 1960s, not the US in the 1700s. They can’t name a single black Kenyan person from that time period?