Decommission old security system

I’ve unfortunately inherited an old Honeywell Galaxy 2-20 C020 system with the house I bought. First few days only the keypad was beeping & you could only hear it inside the house. After a lot of back and forth, I got the code from the old owner to be able to unset the alarm. At this point, I also removed the fuse.

Meanwhile, I phoned the company that installed it (found their number on the outside alarm) and they quoted a ridiculous £180 for a call out or £30/month service contract for 3 years to help remove it or even let me know how to stop the beeping.

And all of a sudden, the loud alarm just went off in the middle of the night with the keypad showing ‘***’ and I think pushing the fuse back in stopped the alarm. The alarm showed a couple of errors, ‘tel line failure & bat mis’.

Found this video on YouTube that I think I’ll follow tomorrow: https://youtu.be/S8O1sruSO2U?si=qPp-6EblQEH5vvyU

Only worry is the 20 minute alarm that’ll ring until the outside alarm’s battery dies off. Is there a way I can stop it? Is there a better way for me to decommission this thing?