Your best and cheapest way to keep the temps in order in a loft in winter is if possible to have a duct connected to the ceiling below and draw in from the house. Built in storage space ie cupboard in a room etc.
Also helps spread the electrical load. With my 150 amp main there is no way I could run all 20kW plus 8 tons at the same time, especially when you add in my veg room and the rest of my home lol.