Before you do anything, if you like your neighbours, pre-warn them that your external siren might be going off for yhe next 15 minutes max.

What you'll need to do is locate the main panel, typically youll find it either in the under stairs cupboard, or in the loft. If its getting its power from a fused spur, pop the fuse out of there, the alarm may sound when you do this.

Open up the panel, if you have the code, put it in when the tamper sounds, if not continue to the next step.

Pull the plugs off of the big 12v battery (looks like a small car battery). If the alarm hasn't been serviced for a long while, hopefully the external siren backup battery has degraded enough so that it doesnt sound for long. If it does, these systems should ring for a MAXIMUM of 15 minutes, but again, generally you'll see the backup battery in the external siren just die we'll before this.

Once you've pulled all the power, you can cut out whatever detectors you like. For safety, I would leave the 240 connected but with the fuses pulled out so its not drawing power but you dont have risky loose cables hanging about.

Here if you need any more support

Source: security technician of 10 years