Do I have to use the company that installed my alarm for monitoring?

No. Installation and monitoring are separate services and can be bought from different companies. In practice they are often bundled because it is simpler, and because police response requires both an approved installer for maintenance and a certificated Alarm Receiving Centre — but you are free to choose each.

Bundling has genuine advantages: one contract, one number to call, and no argument about whether a fault is the panel or the signalling. It is a reasonable default.

The reason to separate them is usually price or service quality. If your installer is good but their monitoring is expensive, or their ARC is unresponsive, you can change one without the other.

The constraint is that for police response you need an approved company holding the maintenance contract as well as a certificated ARC doing the monitoring, so both parties still need to be approved and to work together on the URN. Check before splitting that each is willing to.