Public Servant
if you are going to say something about your work, then you should put your name to it; it is not the private sector.
If he was blogging about crocheting or the state of policing in some other country with no reference to UK policing then he could expect some privacy, but if he is going to blog about people, cases and claim to and work for the police, then he has no right to privacy on those matters.
Things like this should be explained to the Police when they sign up, they are dealing with sensitive information that their public job allows them to be privy to, don't blog about that and expect not to get caught, it is an abuse of position and trust.
Whereas a journalist would be ok dishing the dirt on their newspaper anonymously, but not to revealing a source, there are a different set of ethics involved.