Insulating under the floorboards on your ground floor will save you around £60 a year, and you can seal the gaps between floors and skirting boards to reduce draughts too.
Gaps and draughts around skirting boards and floors are simple to fix yourself with a tube of sealant bought from any DIY store. Floorboards will rot without adequate ventilation, though, so don't block under-floor airbricks in your outside walls.
Older homes are more likely to have suspended timber floors. Timber floors can be insulated by lifting the floorboards and laying mineral wool insulation supported by netting between the joists.
Many homes - especially newer homes - will have a ground floor made of solid concrete. This can be insulated If it needs to be replaced, or can have rigid insulation laid on top.
You don't need to insulate the floors of upstairs rooms in your house if they're above heated spaces (like the living room). But you should think about insulating any floors that are above unheated spaces such as garages, as you could be losing a lot of heat through those.