The ideal temperature setting for a freezer in the UK is -18°C. This temperature is recommended to prevent bacteria from growing and to keep frozen food fresher for longer. While some freezers can operate down to -25°C, it is not recommended to set the temperature too low as it can cause food to lose flavor and moisture. It is also important to not go too cold as it can cause the utility bill to increase. To check the temperature of your freezer, you can use a thermometer and place it between two bags of frozen food, and check back after eight hours to see if the temperature has reached -18°C.