Cut heat loss, then size the heating
The cheapest heat is the heat you don't lose. If your loft has less than 270mm of insulation, start with the loft insulation calculator and the loft insulation guide, which shows how quickly a DIY top-up pays for itself.
Once the house is better insulated, size each room's radiator with the radiator BTU calculator, and check whole-house demand with the boiler size calculator. These are estimates to help you compare options. A heating engineer should do a full heat-loss calculation before a new system or heat pump is fitted.
Measuring tools
Almost every other calculator on this site starts with an area or a conversion. The square metre calculator handles awkward shapes in metres or feet, and the unit converter covers metric and imperial lengths, volumes, weights and slopes, such as the 1-in-60 fall on a patio.