5-10 years ago, I would have snobbily assumed that the mail was merely misrepresenting a decision made for other reasons. I used to think the stories of 'political correctness gone mad' were always exaggerated nonsense (which they often are). I thought that nobody could be as stupid as the tabloids make out.
Unfortunately experience has taught me that there are some people who are determined to push their personal values, however laudable, at the expense of the rights of others and of common sense. Too many of these imbeciles are on 'my' side of the political spectrum- they do not help.
There is a more detailed account from the choirmaster here. A quote from the chief constable:
We need to represent our communities in every aspect of our public presence. Having a male voice choir representing the organisation is incompatible with this
He is, of course, within his rights to ask for the choir to dissociate from the police- apparently, they don't have any serving officers left. He's still a fuckwit for doing it.