Removing Home Alarm System (UK)
I've done something similar recently. Our system was wired into the same fuse at the consumer unit as the upstairs lighting circuit (despite the control box being downstairs). It was powered via a fused spur located in the under stairs cupboard which is adjacent to the control panel in the hallway. I can't say if this is typical but it might give you some idea where to look.

The problem was that the control panel and the alarm box on the outside of the house both have backup batteries, and if you tamper with the system the alarm will go off, or if you leave the power off too long the alarm will go off. To avoid this, I put the ladder up to the alarm box on the front of the house and disconnected the battery. I had someone on standby to key in the code in case it triggered the alarm. Then I cut the power to the control panel by removing the fuse in the cupboard and disconnected that battery too. This is what I understood from the alarm company to be the proper procedure. Then I started cutting wires to the sensors and the wire that goes to the siren.

Possibly, in your case, the system has been disabled for so long that the batteries are flat anyway, but you still need to confirm that you've properly removed mains power from the control panel. If you really can't find a switch or fuse for it, you could switch off power to the whole house, open up the control panel, remove the mains wires and cap them with a choc block. Safer than working with a wire that connects to who-knows-where and could become live at any time.