Old heater radiators can be hard to get rid of. Considering the weight, it seems like a lot of metal to merely throw out and not recycle. Radiators can definitely be recycled as scrap metal. Getting such a heavy load out the door and to a recycler is easier than it sounds.
How to dispose of old radiators
Tip the disconnected radiator to make sure all water or oil has emptied out of the holes where the input/output pipe(s) were attached. You'll want the radiator to be as light as possible before moving it.
Move it out of the building and onto whatever mode of transportation you'll be using to get it to a recycler. Use a handcart on level parts of the trip. If you have to slide the radiator down the stairs because of the heavy weight, protect the stairs by laying rows of 2x4's down the stairs to make tracks. Several planks will probably be needed to reach the top. (See the video link below.) You may want someone at the bottom of the stairs to keep the bottom planks from slipping out. Tie rope around the radiator and let it slide down the planks as you let the rope out from your position at the top of the stairs.
Look up a local scrap metal dealer who accepts radiators. You may be able to get from 25 to 40 pence a pound for aluminium or copper radiators. Copper tends to pay more.