The correct way to remove the white cable is to turn off the power, open the fusebox and disconnect the cable in there.
Note that there will still be a live terminal inside the box even with the switch off.
That fusebox is also overloaded, that 40A MCB should not be in there.
flameport, you're a star! Thank you so much - it took me about five minutes, including removing the actual alarm box (which was the hardest part). I must admit, I had no idea what the inside of the fuse box looked like, but that was all pretty simple once you've got to info from the experts! I've already put shelving over the site of the box.
I'm surprised that it's overloaded though - the last two things that have been done to it were installing a shower (maybe ten years ago) and a change when an oven was installed (maybe five years ago), both were done by electricians.
A great big thanks to everyone that's offered advice on here - you're great.
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