Or just leave the external battery powered bell to run down - won't take long.Anybody with a few tools and a ladder could disconnect it for you. When they turn off the mains power and disconnect the alarm panel battery the self activated siren in the outside alarm siren box will probably sound. They will need to climb up and open the siren box and cut the wire to the siren to silence it.
Bear in mind that those restrictions only apply to installers operating as a business. There's absolutely nothing to stop you from fitting or removing your own alarm in your own premises. And removing an alarm is a very easy job.There are legal restrictions on who can fit an alarm. I've no idea if these apply to removal of an alarm.
That is because the alarm panel battery is dead. When the mains electricity is cut off and the alarm panel battery is dead the self activated siren in the outside alarm siren box sees no 12 volt feed from the panel and starts to sound. You could cure this problem by fitting a new alarm panel battery.The alarm seems to go off when there is the electricity goes off but its never been on