I'm in a middle-terrace house with relatively modern windows and insulation, so I'm lucky that there's only really a few places that heat can actually leak out.

Basically all I do is use a draft excluder for the gap under the front door and be selective about opening windows. Then I basically set the thermostat to 19/20 degrees (and the timer on the boiler set to stop it coming on overnight) and that keeps things warm enough without the heating needing to come on for that long during the day. Obviously in the middle of winter it needs to come on a bit longer, but those would be the days I'd be using the heating whatever approach I went for.

Weirdly I half do it for the house plants, I can always throw a jumper on to a point, but I've got a few plants that really don't seem to like the cold. Might as well make it comfortable for all of us if my house doesn't cost a bomb to keep at a good level.