Can I get police response without paying for monitoring?

No. Police response to an alarm activation requires a URN, and a URN requires the system to be monitored by a certificated Alarm Receiving Centre under a maintenance contract with an approved installer. There is no route to police response for an unmonitored or self-monitored system.

This trips people up because the alternative sounds reasonable: if my app tells me the alarm has gone off, why can I not just have the police attend on that basis?

The answer is that the police are agreeing in advance to act on signals from a system they have conditions over — installed to standard, maintained, monitored by an inspected centre, with a false-alarm record they can hold someone accountable for. A self-monitored system offers none of that.

What you can always do is report a crime yourself. If your app shows someone in your property, that is a report of a burglary in progress and the police will respond to it as such. The difference is that you are the verification step, and you must be awake, in signal, and looking at your phone.