Range from the cheapest Fit a radiator £148 (Capital Maintenance & Installation Limited T/A Gas Pro CO based in SW4) to the dearest Fit a radiator £220 (PAV Heating and Plumbing based in SW17).
Comparing heating engineers estimates for fitting a new radiator in SW12 gives you an idea of who will be better value, but the best way to compare costs is to get actual quotes. Radiators vary in price, and additional hours will be required to make pipework changes.
What to do - Get rates and quotes for removing your old radiator and replacing it with a new radiator by posting your job to heating engineers in SW12.

"Fit a radiator"
BATTERSEA
"Fit a radiator - radiator was removed for plastering wall behind. Needs to be refitted."
SW8