In the UK it is not actually legal to refuse to serve members of a particular group, Geezer. Establishments cannot discriminate on the grounds of (among many other things) gender, or race. The “Management refuses the right to refuse...” etc. would fall foul of anti-discrimination rules if it could be shown that a significant majority of the people individually barred were, say, black.
The National Black Police Association does not fall foul of such legislation because it does not discriminate on the grounds of race or skin colour when it comes to potential members. From its website:
“The NBPA is open to all in policing on application, there is no bar to membership based on colour.”
“The definition of "Black" does not refer to skin colour. The emphasis is on the common experience and determination of people of African, African-Caribbean and Asian origin to oppose the effects of racism.”
Quite how a “National White Police Association” would be received if it were formed is not quite clear. It is doubtful that any black police officers would want to join such an organisation and similarly, as far as I know, the NBPA is not overrun with white applicants.