You only have to look at the ONS Census for a pretty obvious clue that while there is diviersity even within ‘White’, ‘Black’, ‘Asian’ etc. there are clearly defined groups that are used for all sorts of statistical analysis on the population:
England and Wales Northern Ireland Scotland
White
English/Welsh/Scottish/Northern Irish/British Scottish
Other British
Irish
Irish Traveller Gypsy or Irish
Any other White ethnic group, please describe
Mixed / multiple ethnic groups Mixed or Multiple ethnic groups
White and Black Caribbean
White and Black African
Any Mixed or Multiple ethnic groups, please describe
Asian / Asian British Asian, Asian Scottish or Asian British
Indian, Indian Scottish or Indian British
Pakistani, Pakistani Scottish or Pakistani British
Bangladeshi, Bangladeshi Scottish or Bangladeshi British
Chinese, Chinese Scottish or Chinese British
Any other Asian, please describe
Black / African / Caribbean / Black British
African, African Scottish or African British
Any other African, please describe
Caribbean, Caribbean Scottish or Caribbean British
Black, Black Scottish or Black British
Any other Black / African / Caribbean background
Other ethnic group
Arab Arab
Arab Scottish or Arab British
Any other ethnic group, please describe
Biologically they may all be the same but people self-identify with these groups.
I don’t really see what the problem is?