Ours is from mid-1700’s. But it’s very small so easy to heat, therefore toasty warm.
Loft insulation - it made a huge difference and isn’t particularly disruptive or expensive to install. You’ll need to do all loft spaces for best effect.
We have double glazing. You could install triple glazing or secondary glazing. Thick interlined curtains make a really noticeable difference, the temperature difference behind them is obvious. Honestly you need to invest in decent windows (they’ll be expensive for wooden sash).
Slightly depends on layout - the more external walls the room has, the colder it’ll be. So a long narrow house will be colder than a squarer one. Ours is an odd shape so some rooms are chillier than you’d expect!
Prevailing wind, where windows are, shelter from the wind, north-facing rooms, being in a hollow... all those things impact how warm it’ll be, but probably don’t make it into the energy rating (?).
Fireplaces - use them or loose them, open fires generally have drafty chimneys, so consider fitting wood burners.
How do you use the house? If you’ve rooms that are excessively big for number o occupants then obviously it’s not going to be efficient use of resources. I know it’s super-old-fashioned, but having everyday room that’s “big enough” rather than massive cavernous space will mean you can heat it properly without feeling like you’re just setting fire to £10 notes.