When I first decorated the house I now live in, I put a thin frame the same size of the radiators on the wall.
This was done cheaply with the cheap mouldings you can buy from any diy shop. I then painted the frames to match the existing woodwork. I put radiator heat reflectors ( a sort of very thick tin foil available from diy stores) inside the frames. These reflectors can hardly be seen with the radiators in place.
As well as saving heating costs, the radiators can now be left in place during decorating. The wooden frame is easy to paint or varnish without removing the radiators.