Back up battery obviously dead. Should be an unswitched fused spur adjacent to alarm panel. Alarm panel is obviously on the lighting circuit, remove fuse from the fused spur which will allow you to turn on lighting circuit again. If no fused spur then you could remove the internal fuse in the control panel this would prevent the panel powering up. Better though if panel is wired direct to the consumer unit then disconnect it there. unless you are competent, you will need the services of a competent person to disconnect the wiring at the consumer unit.