You can get an idea of the price of radiators online from Wickes or Screwfix.
The actual fitting of them is not a big job. My friendly neighbourhood plumber recently charged me less than £150 for fitting 3 replacement radiators.
Get a personal recommendation from a friend or relative if you can.
Not wishing to be in any way sexist, I would hazard a guess that the OP, unlike Siggy or myself, doesn't count a Stillsons among her prized possessions!
A slightly more serious reply to Siggy:
I find that Stillsons, with their serrated jaws, can chew up the nuts on the radiator tails and valves. Better to use flat jawed moveable spanners, methinks.