I have an air source system with all the right things thought about, however, upstairs radiators are too small in the master bedroom (coupled with floor to ceiling north facing gable end window), the joinery was shite - poor windows and doors (glazing units failed, door seals piss air through). There is a air exchange heat recovery system installed but I switched it off as sucking 23 1/2 hours worth of cold air out of bathrooms to exchange it with external cold air and pumping it into other rooms is a nonsense in a domestic property.
We're moving to a similar sized property soon with solar water heaters and oil boiler, I'm confident my heating bills will reduce by 30 to 50%
Cheap heating in summer (even NE Scotland summer) doesn't really help in the UK in my opinion
If I believe the EPC calculation for my new property it states I need 22,000 kWh for heating per year (I know this is high as my current property EPC suggests I need 22,558 kWh and we only use ~18,000) that would translate to approx. 2,400 litres of heating oil (assuming 90% efficiency of oil fired condensing boiler).
So even if oil was to cost £1/litre (which it doesn't) I'm going to be a lot better off