So basically, Sandy, one white person (say a Bulgarian or a Pole) for each black person. It's strange that white people can be seen as a "fairly homogenous group". Try saying that about black people.
“What happens if for every 100 new applications, only 20 come from people from ethnic minorities?”
To answer that question simply see what has happened (and what would probably still be happening if they were recruiting at all) in the Fire Service, SP. I know this article is a bit old and from the Daily Telegraph (which is probably only just second in some people’s mind to the Mail in terms of sensationalism):
http://www.telegraph.co.uk/news/uknews/1576499/Fire-brigade-discriminating-against-white-men.html
But there is definitely truth in the article. Of course the Fire Service’s Chief Officer refuted the claims saying that because personnel were not actually recruited at the sessions then no discrimination was taking place. So I imagine it would have been perfectly OK if the sessions were restricted to white people only.
As far as the London Fire Service goes I know that they have been adopting a similar policy in their recruitment. They are not recruiting at the moment but when they last did white males need not have bothered applying (but of course you won‘t find that written down anywhere).
I really don’t know what it is about public services having to be representative of the communities they serve to such a degree that ethnic minorities must be targeted in this way. If they don’t want to become police officers or firefighters then that’s that. Just leave the jobs to those who want them and avoid the bad feeling that such discrimination (or call it what you like) brings.