I grew up in a house without central heating and still don't see it as essential, but perhaps I'm just quite tolerant of cold.
Layers of clothes are my first go-to, indoors or outside. Blankets and throws when sitting still - even heated throws only cost pence to use. Hot water bottles in bed or on the sofa - heat the bed, not the room. Hot drinks, hot food. And movement - get outside for a brisk walk if possible, but blitzing around with the vacuum cleaner is warming, so is ironing. They both use electricity though, so what about sweeping and dusting with some vigour! Indoor exercise if you can't get out - from stretches and lunges, lifting weights (a can of tomatoes in each hand will do), to skipping, push-ups, crunches, star jumps, or just dancing.
Close the curtains before it's dark. Lined curtains are best and it's easy to attach a fleece blanket behind them with safety pins - who's going to care, when it's dark? Cling film smoothed over the window pane really helps keep heat in, as does bubble wrap (the sheets in the supermarket vegetable crates are the right size for my windows and I've always been allowed to take some).
I boil a full kettle in the morning, make a hot drink and put the rest of the hot water into a big flask, which I wrap in a folded towel. I tend to leave my drinks until lukewarm anyway but even by bedtime, the flask is still warm enough to make an instant hot chocolate, and if I tuck my hands under the towel it warms them too.
To save electricity, switch of at the wall wherever possible. I have a battery clock in the kitchen so there's no need to have the oven display beaming the time at me, nor the fan going after I've emptied the oven, it gets switched off immediately at the wall, as do the hob, kettle, toaster and microwave, dishwasher and washing machine, TV and Sky box (unless Sky is set to record something after I've gone to bed). Phone chargers should always be switched off at the wall for safety, if a plug is warm it's using electricity.
The appliances that do need to stay on are still quite a few: fridge and freezer, the router, Echo (Alexa), the boiler programmer, my sunrise alarm clock.
I have some solar lamps which charge very well during the day in a south-facing window, and are bright enough to mean I don't need to switch a light on to watch TV, or use my laptop or phone.
I have an open fireplace - will get a more efficient stove with a door if I ever come into some money - and I gather fallen wood all year round when I'm out with the dog, stack outside for however many months or years needed to dry out, then cut and stored in the shed until there's room by the hearth. Keep the warmth in the room by closing doors and using stuffed 'snake' draught excluders, lined curtains across all external doors, and a circle of card attached with blu-tak or tape to cover keyholes. Insulate pipes and drape spare duvets and sleeping bags over the hot water cylinder as extra insulation, but don't seal the house completely or condensation becomes a real problem.
My adult offspring have all left home but in their teens they would walk around barefoot in t-shirts, cranking the heating up to 25° before I got home from work and had a rant about it, day after day. Now they're paying for their own heating I think at least one of them is taking some tips on board!
Oh, my last tip - cuddly cats and dogs are very warm...