Themostat

Your room thermostat should be set to the lowest comfortable temperature, typically between 18 and 21 degrees. You don't need to turn your thermostat up when it is colder outside; the house will heat up to the set temperature regardless.

https://www.warmup.co.uk/blog/5-mistakes-people-make-with-their-thermostat

Could I save money by controlling room temperature?

The colder you keep your home, the lower your heating bill will be – simply because a cooler house loses less heat. Of course the better your insulation is, the more warmth you will get for your heating buck. And as we’ve seen already, living in a home with living areas colder than 18°C is not a good idea for your health. So is it possible to save money by tolerating a slightly cooler home? Very much so.

In the graph below we have provided some estimates for how much a typical home might save by pursuing simple energy-saving behaviours. Please note these are only estimates of pursuing each behaviour in isolation. The biggest estimated saving is ‎£139 a year by reducing the thermostat setting from 20°C to 18°C. Of course that might be too much of a change for you, so we’ve included a few other ideas.