Range from the cheapest Fit a radiator £120 (Touch of Heat based in E1) to the dearest Fit a radiator £220 (RD plumbing and heating services ltd based in SE15).
Comparing heating engineers estimates for fitting a new radiator in SE17 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 SE17.

"Change one radiator to install a new own in place"
SE1