Smashed back of Iphone, how much to get replaced in UK?

Me being the clumsy idiot that I'm, I've smashed half of the back of my Iphone 4. I was going to go to Apple to get it replaced but I can't really afford a lot. I've done some research and people have said that its usually about £20 or nothing at all? I live in the Uk, if that makes any difference. Also, I could order replacement glass off the internet, but I'm not sure if they are legit.

You only need the glass lens, don't you? I found this for you which is only $10, you can have a look

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