"Is there a police box in this town?"
Is there a purpose to the particle に here? Couldn't the particles be は and が respectively (even in question format)?
There is an unwritten grammar rule that says は cannot appear more than once within the same sentence. Also, に is paired to あります for the sentence structure; "place / item / object " に ある for existence of something or an location
To simplify further,
まち に ~ ありますか means "Is there ~ at, in the town ？"