I love this question @LornaLove. We all have such different needs and experiences and I’m learning things (the info about the air con units is fascinating).
Like you in your earlier life, I live in the sub-tropics so my house isn’t set up for cold weather. Big windows, and open, airy spaces. Not great when it gets colder for a couple of months of the year. Because I don’t like being too cold or too hot, we have lots of methods to keep our environment comfortable.
We have wood-burning slow combustion heaters at either end of our long home. We don’t use them a lot, and often we only end up using one of them, but we will have one burning all day for a month or two. Really, this is more about aesthetics than the economic heat production. I just love a good fire!
We also have two reverse cycle, split air conditioners at either end of the house. So if the temp drops but we don’t want to light the fire, we can change the air con from cool to heat and warm the place up – or even just warm the area we’re actually in. We can close off some areas to save on cooling/heating the whole house.
We have blinds on the windows. We have some big window areas. This keeps the warm in during winter but helps to keep the heat out in summer. I’d recommend having heavy curtains in the Northern part of the UK. That will help reduce heat loss through your windows and so cut costs.
Make sure your loft is insulated. Ours is. That helps prevent heat loss through the ceiling/keeps the heat out in summer. It really does make a difference.
Even using door snakes can help. As @Call_Me_Jay suggested with your doors, you want to reduce any heat escaping through gaps around your doors/windows/ceilings.
I also have throws on the lounges for when it’s just a bit cool at night. I bought a couple of heated blankets too. When I’m working during the day in winter, I might not want to heat the whole room but because I’m sitting and not moving, I can get cold. I just plug one of those in and it saves heating the whole room.
We have solar panels for electricity generation and a solar hot water system, so our bills are now very reasonable. Electricity is expensive in Australia. Solar power helps negate those costs. It costs a bit to put the panels on the roof, but there used to be great rebates and we actually get paid more for any unused power that goes back to the grid than we do for the power we draw from the grid.