Its possible to seal the whole window off, B&Q sells it. Not advisable for most people or anyone who wants to open the window but old and poor its a common trick. In USA they call it ghetto wrap apparently, obviously richest country in the world has some extremely poor old people our lot are probably minted in comparison. I understood this much just from watching yt vids on anthracite burners, they get given half a ton of bags free when buying one its that cheap and available so bit of a god send in those very cold parts of usa.
Can always go back to warming water over the stove. Wales gives free planning permission in green belt to those who have no need to connect to the electric grid at all, so there has to be a whole section of study over extreme heating methods