Old recessed radiators with reflector foil
If you want to make the effort, you could fish the reflectors out, clean them, and reinstall them. But vacuuming the radiators may make as much or more difference...

(The idea behind these is supposed to be to encourage more heat to go into the room, rather than heating the wall. I don't know whether anyone has actually measured whether this makes a difference or not, or if putting non-reflective insulation there would work just as well or better...)