Reflective panels slipped behind a radiator are one of those cheap measures that sound like a gimmick but have a real, if modest, basis. The idea is to bounce heat back into the room instead of letting it soak into the wall behind, and on the right wall it does just that.
The principle
A radiator warms the wall directly behind it as well as the room. If that wall is an external one, much of that heat then escapes outside. A reflective panel, foil-faced board or even good-quality kitchen foil on card, reflects the radiant heat back into the room rather than letting it warm the bricks and disappear. The effect is small per radiator but the cost is tiny, so the sums work out in its favour.
Only behind external walls
This is the part people miss: reflectors only help behind radiators on external walls, the ones with cold outdoors on the far side. A radiator on an internal wall is warming another part of your own house, so reflecting that heat back saves nothing worth bothering with. Walk round and note which radiators sit against outside walls, and treat only those.
Fitting it cheaply
You can buy purpose-made panels with a foil face and a thin insulating backing, which are easy to slide down behind the radiator on brackets or tabs. Or you can make your own from rigid foil-faced insulation board cut to size. Either way it tucks out of sight, needs no tools beyond scissors or a knife, and once it is in you forget about it.
Keep it in proportion
Reflectors are a genuine but minor saving, firmly in the every-little-helps category rather than the game-changers. They are most worthwhile in older, poorly insulated solid-wall homes where the wall behind is doing little to hold heat. If your walls are already well insulated there is little for a reflector to claw back, so spend your effort on the bigger fabric measures first and treat reflectors as a cheap finishing touch.