Then he'll have to pay the extra £500/month. Call it Greta's Revenge. There will come a time when we think of it as Greta's Gift to Humanity.
More seriously, this talk of £660pcm is pretty meaningless because the tariff is a huge factor in it. It would be more helpful if we knew how much actual energy (in kWh) is being consumed. I posted a couple of weeks back on our own gas consumption. These are daily average figures for each quarter:
Three months to November 2020 - 25.4 kWh
Three months to February 2021 - 64 kWh
Three months to May 2021 - 53.9 kWh
Three months to August 2021 - 18.4 kWh
This is a detached house, roughly same size at 1360 square feet, but new (2017) build and well-insulated. Two adults and two young children at home all day. Gas provides both heating and hot water from a condensing boiler, new in 2017.
The thermostat is set for 22 celsius upstairs for when get up and 21 celsius in the evening for children's bath- and bed times. Downstairs it's set to 21.5 until 0900 in the morning, and 22 from 1600 in the afternoon and evening when we tend to be a bit more sedentary. The system won't let the temperature fall below 19 during the day downstairs, or 17 upstairs. So far this winter the radiators feel cool to the touch more often than hot. It's also worth pointing out that these are thermostat settings, and the temperature in our living room, for example, is usually a degree lower than where the thermostat is actually located.
Personally, I would rather have it slightly warmer: I come from a tropical place where 20 celsius is a cold winter's day. But with a vest, long sleeve shirt and sweater, plus moving around chasing the DCs for much of the day, 20-21 is easily manageable and I don't feel cheated in any way.
However, I would not want it much colder than that during the day. Putting another sweater on just doesn't cut it when your hands are so numb that you cannot hold a kitchen knife safely, or change a nappy, or click a mouse with any kind of control.
There's a reason why government guidelines recommend a minimum daytime of 18 celsius even with appropriate clothing - less than that is demonstrably harmful to health, with actual evidence to back the recommendations up. Sitting around in 15/16 degrees in thermals, as some on here would suggest, is utter nonsense. It's harmful, and while your wallet may benefit in the short term the cost gets picked up by the NHS instead in respiratory conditions, flu, strokes and heart attacks, so it's irresponsible too.
Walking around half naked while you superheat your house to 24 or 25 celsius is equally wasteful and irresponsible if you give the slightest shit about climate change.