2. Insulating solid walls isn't straightforward and may be something you want to leave to the professionals, although internal wall insulation can be fitted by DIYers. Perhaps the easiest method is to use insulated plasterboard, which comes in different thicknesses. Another option is to build a stud wall along the exterior wall and fill that with insulation. This will make the room a bit smaller and, in both cases, you'll have to remove and then refit or replace the skirting boards, door frames, coving, etc, so it's a lot of work and disruption. For this reason, insulating walls internally is often best left until you're renovating the room.
4. If insulating your home's exterior walls seems like too much work and expense, there is a much easier way to reduce heat loss through them. Radiators tend to be on outside walls because they're usually fitted under windows, which means the heat from the radiators can go outside into the cold as well as into the room. Fitting radiator foil behind the radiators will help to stop heat escaping through the walls, making it feel warmer indoors.