A maximum of 18 oC is usually adequate for most people. Simple, relatively cheap measures like putting draft excluders around windows and doors and insulating the loft are often more effective than costlier options like double-glazing. But if you are stuck with conventional means, then gas central-heating is probably the least damaging, most efficient and cheapest. Oil is dirtier than gas and incurs transport costs. Coal is the worst primary fuel for global warming and acid rain and open coal fires are appallingly inefficient.
Electricity is expensive and is often uncleanly generated.
Even in a cool climate it can provide 40-60 per cent of hot water required for an average house. Insulating hot water pipes also helps. Gas appliances are almost always more efficient, cleaner and cheaper to run than electrical ones. Microwave ovens use little energy compared to a cooker - but any energy saving is totally lost if you buy frozen convenience food to put in it. Pressure cookers are good and jug kettles better than others.