Your fucked with your scenario the house is not retaining any heat in such a big house as well. This year I insulated our loft and boarded it out, converted the attached garage to a room with loads of insulation, installed a log burner. Ive done some experiments the heating on for 2 hours in the morning and two at night, the house unless it’s Baltic is around 16 degrees, I set the thermostat at 19 degrees and put in hallway with radiator no trv, every other room has TRVs. We have new double glazing as well. My gas bill for the two periods is around £3 fir the day as boiler heats up to 19 then shuts down, so I then tried sticking it on all day that’s 6am-9pm (we are always home) cost £4-5) depending on how cold it is outside. We are more than warm and for the extra few £1.50 it’s worth it. Alternatively we stick the log burner on around 11am if it’s cold until 6pm however thats a lot of logs so probably £3 in logs plus the £3 for gas it’s actually cheaper to just use gas.
Maybe try trvs and turn them down in the rooms you don’t use that much, double glazing would make a difference and definitely insulate your loft, it’s piss easy to do send cheap if you do yourself, just wear a mask and white suit as that stuff is itchy as hell.