The Black Police Association's own website states:

"The NBPA is open to all in policing on application and there is no bar to membership based on colour".

I would have thought it was illegal to prevent people joining any group on the grounds of their race in the UK. Race can mean black people, but it means white people too. Most "minority groups" allow any sympathetic people to join them. Many white people were involved in the American Black Civil Rights Movement.

If it’s true that the BPA does not permit white people to have full membership, (and Wikipedia is hardly an authoritative source), then it is stupid to exclude people who are sympathetic and helpful to their cause.