I went through all this when I wanted a freezer for an unheated outhouse in Cumbria, where we could get temps down to -20C or lower.
I found two which were rated to -10C. One was the Currys’ own brand, which was not very high quality (a friend had one), and the other was the Miele. I spoke to both manufacturers about the situation, and Miele were the most helpful. As far as I remember, they couldn’t warrant it below -10C, but didn’t seem to think it would come to any harm, and did seem to be sure it would work again once the temperature rose again. We agreed that at -18C ambient and lower, nothing would defrost anyway! Lol.
I still have the Miele, now in a North Cornish barn, and going strong. I must have had it seven years or so now, I would think.