I have never, not ever, had to turn my heating up in December because it’s been set at 19°C since September. I don’t understand this acclimatising theory at all. My experience is the opposite. In summer I turn my heating off, because although the house might drop to 18°c, that feels lovely and cool after a 30°C day. I don’t want my heating kicking in then. But 18°C will feel cold if it’s only 14°C outside, for sure.
Sometime in September, I’ll take it from the off setting to 18°C, 19°C or 20°C for the times we are using each room and are up. And then I ignore it. Very occasionally, I might be unwell or feel cold for some other reason and boost it, but it’s rare. The heating will be on longer in December, because it takes longer to heat a room from 1°c to 19°c, not because 19°c isn’t warm enough, and it doesn’t need to be any higher. 19°C is fine all winter.
This year I turned it up from ‘off ‘ later, because my budget to get through relies on me not using my credit until November. I can use my full DD in October, but was hoping to keep building a bit in September. It’s better putting up with 17°C for a few hours here and there now, than burn through my credit and have to cope with 12°C inside in February because I ran out of credit too soon and can’t find spare income for the gas bill. I don’t usually do it, but it usually have more income to start with!