Nah, in that case Canada wins. From the CIA Fact-book...
UK (population) 60,776,238 (July 2007 est.)
Ethnic Groups: white (of which English 83.6%, Scottish 8.6%, Welsh 4.9%, Northern Irish 2.9%) 92.1%, black 2%, Indian 1.8%, Pakistani 1.3%, mixed 1.2%, other 1.6% (2001 census)
USA - 301,139,947
white 81.7%, black 12.9%, Asian 4.2%, Amerindian and Alaska native 1%, native Hawaiian and other Pacific islander 0.2% (2003 est.)
note: a separate listing for Hispanic is not included because the US Census Bureau considers Hispanic to mean a person of Latin American descent (including persons of Cuban, Mexican, or Puerto Rican origin) living in the US who may be of any race or ethnic group (white, black, Asian, etc.) Self-identified hispanic population, about 12.5%, which would shrink the white group to the low 70% as most ID as white, not black, except the Cuban population.
Canada - 33,390,141
Ethnic groups: British Isles origin 28%, French origin 23%, other European 15%, Amerindian 2%, other, mostly Asian, African, Arab 6%, mixed background 26%