Read the title of the thread. While half of Britons can't name a black historical figure, I bet 99% of Americans can name a Black British historical figure either. I sure can't.
Ugh George Washington Carver is known by a LOT OF people. Not to mention we have days dedicated to some of the greatest historical black Americans, Martin Luther King Jr for instance. But outside of that nearly every American knows a decent amount.
Even the hateful useless Americans know President Barack Obama.
Others of note when I asked several friends at work (blue collar average Joes and Jills, hotel maintenance):
Booker T Washington
Rosa Parks
Harriet Tubman
Jackie Robinson
Malcolm X
Hattie McDaniel
Fredrick Douglass
Oprah Winfrey (yeah I don’t know if this counts but I included it)
Out of the 15 or so people I asked all of them said Obama and MLK Jr.
Lots of Americans know Crispis Attucks, he's covered in history class. They may not associate him with britian but he was shot years before the revolution so he was a British subject.