I think high street shops will charge about £50 for this sort of repair, or you can use the Ebay services roongood mentioned. (I would delete all the data on the phone before sending it to them)
However both are only good if the actual LCD under the screen glass isn't cracked, and all connections inside are still OK. Also the touchscreen you'll get is not original from Apple and there is always a difference in sensitivity. Finally any other damage (eg cosmetic, etc) is not covered either.
The other option like premaru said is out of warranty service at Apple for £139. This will get you a shiny refurbished unit essentially indistinguishable from new, plus it also comes with a 3 months' warranty.