How do I bathroom / kitchen / extension? etc.

Our living room is pretty much the same and it's basically fine (although sofa is on legs and has angled back which allows airflow). We have a shelf over the radiator behind sofa, which is surprisingly handy.

I've got exact same setup as you (1920s house) and we left the (small) rad behind the sofa to avoid the pain of putting it under the window. if it ever goes on, it doesn't really heat the room sadly. think it all just goes into the back of the sofa. (it's not an issue in our room thankfully because we have a wood burner, it's relatively small and enclosed, and it's also south facing so gets full solar gain.)