Isn't this a AE/BE thing? Don't Americans treat "police" as singular? In any case, if the police "is coming" then the police "comes", whereas if the police "are coming" then the police "come". I think all four options should be accepted, but would a native English speaker (from either side of the pond) comment on that.
I'm an American. My understanding of British English is that they often use plural verbs with organization names but Americans would use singular verbs for them. Maybe that's what you're thinking of. An American might say "NASA is still number one in moon landings!", but a British person might say "Nasa are still number one in moon landings." I think the American logic is that the organization is syntactically a singular noun, and the British logic is that it is semantically a group of people.
For whatever reason, "police" is a plural noun in any usage I'm familiar with.