Any kind of reflector our insulation and reflector or insulation between the rad and brick is going to put more heat in the room and less in the wall. How much is not clear. How many radiators are we talking about?

You could install one and see what it does. If the wall behind the rad is cooler with the panel in place than without it, you are doing something.

I think that foam/foil panels are a good idea for cost and ease.

I think the fins are a waste of money. They won't increase efficiency. If you needed to warm up a room because it did not have sufficient radiators in it compared to other rooms, it might be worth a look, but a fan blowing lightly on the radiator would probably work just as well.

1. Fins alow radiator to warm the area quicker with little or no long term energy savings. No idea where to buy them.
2. A reflector makes a person feel warmer in perportion to distance they are from the radiator. This is the result of air tempature within the room plus heat radiating onto the body from reflector. A reflector would result in energy savings because room tempature can be lowered while a person receiving radiant heat remains comfortable. The foil faced foam should work as long as its not closer than 1 or 2 feet of radiator. 2 feet from a fuel burning heater could possibly damage foam.