Part of it is boring: you generally don't join the police force of a foreign country or a "foreign" country, nor over a certain age, nor in a country where you do not perfectly speak the language. So it is fairer to compare the proportion of police of a certain group with the proportion of that group of those people who were born in this country. Non-whites are a greater proportion of the total population than they are of the population that was born here.
Part of it is aptitude: blacks have low aptitude. Police officer is a decent job. Blacks are underrepresented in all decent jobs.
Part of it is cultural: many Muslims do not recognise British law, so have little interest in enforcing it. Many blacks identify more strongly with criminals (e.g. Mark Duggan) than with the police.
I very much doubt that the police discriminates against non-whites; I find it much more likely that these is discrimination in their favour.