Note the deterministic forecast is the most likely outcome, and there is uncertainty around the forecast, so 5C means the actual coule be a bit lower or a bit higher. Note also that the forecast temperature refers to screen temperature, that is, the temperature in a Stephenson screen which is 1.5m above the ground. The ground minimum temperature will be lower on a clear still night. Also note that the temperatures you see on weather maps usually refer to towns and cities. Rural areas will be a degree or three lower. My potatoes got damaged by frost during the last cold spell even though the Met Office forecast minimum was 4C; my allotment is on the outer edge of town, and lies between fields and housing estates, so the minimum there likely got lower than the urban areas.
