The only real way to get into it these days is by volunteering as a special constable and then moving onto a full time policing course after you have a bit of experience. Hopefully then you can start as a salaried officer soon after. It's a tricky business to get into. Ethnicity shouldn't come into it, the reason you see fewer ethnic officers in many areas is that racism was rampant in many forces right up until about 20 years ago (c.f. Stephen Laurence case) so there is still a large resentment of the police in ethnic cultures.