Floors get overlooked because you cannot see the heat leaving through them, but a ground floor sits against cold earth or a ventilated void, and in an older house it can be a real source of both heat loss and the draughts that make a room feel chilly however high the thermostat goes.
Two kinds of floor
A suspended timber floor, common in older homes, has floorboards over a ventilated gap, and you can often feel air moving up between the boards on a windy day. A solid floor, typical of newer builds, is concrete laid on the ground. The two need different treatment: the timber kind can be insulated from below if there is a cellar or crawl space, or from above by lifting the boards, while a solid floor is usually only insulated as part of a major refurbishment when a new screed goes down.
The quick wins first
Before any major work, deal with the draughts cheaply. Filling the gaps between floorboards and along the skirting with sealant, a flexible filler or proprietary strips stops the worst of the cold air coming up, and a rug over bare boards adds both comfort and a little insulation for the price of a rug. These small steps take the edge off long before you contemplate lifting a floor.
The fuller job
For a suspended floor with access underneath, insulation can be supported between the joists from below, which is the least disruptive route as the room above stays intact. Where there is no access, the boards are lifted, insulation laid in, and the boards relaid. It is more work than a loft but follows the same logic, putting a warm layer between you and the cold, and it pairs well with rewiring or replumbing when the floor is up anyway.
Keep the air moving below
A suspended timber floor relies on those underfloor air bricks to stay dry and rot-free, so never block them in the name of stopping draughts. The job is to insulate above the ventilated void, not to seal the void itself. Done properly, the timbers stay dry and the room above stops feeling like it has a cold breath rising through it.