The best setting for your central heating will depend on how comfortable you want the room temperature to be. According to the Energy Saving Trust, this is typically between 18°C and 21°C[1].
You should ideally set your thermostat to the lowest comfortable temperature. This way, the rooms of your home, assuming you have the right radiators in each room, will stay at a comfortable setting throughout the day.
[1] https://energysavingtrust.org.uk/take-control-your-heating-home/