jj109, this question has been posted in the law section and is not asking for your opinion.
The law states that it is illegal for a dog to be dangerously out of control in a public place.
The law states that a dog is dangerously out of control if it attacks someone's animal.
https://www.gov.uk/control-dog-public/overview
There has been cases where a cat has been killed and the law has deemed that no offence has been committed because it is a dog's natural instinct.
However, for an offence to be committed, it would need to be proved that a human had encouraged an animal to attack another animal, declined to take reasonable steps to prevent the attack, or had allowed their own animal to be dangerously out of control in a public place.
So, if you or any other witness saw that the owner did nothing to prevent the attack then an offence has been committed. It could have happened so quickly that there was nothing the dog owner could do.
I suggest you call the police again and see if a different officer has a different opinion. As always with the law, there are grey areas.