off topic

I have never done it myself as I always have good cases for my phones but I think you may have to go to a phone shop, there are a few around Medway, If it is still there then there is one opposite Sports Direct in Gillingham next to a bed shop.
 
Lots of places will do it if you ask. is it the front or back screen? (iPhone 4) because the back is very easy to replace and a lot cheaper if you do it yourself. The front however probably isn't worth risking, however, there are guides on youtube to help you.