I would not bother with replacing the glass and digitalizer only, especially not at £20. They go for about £6-9 on eBay. After watching vids on Youtube it takes ages and it looks like a massive faff.
Instead save yourself some time, get a full replacement screen assembly (lcd+glass+digitlizer all built and ready to be connected) off eBay for £30. It takes less than 5 minutes to swap it over. Just buy from a good seller who will offer warranty as backlight on the one I bough did not work but sorted it out with the seller and got a replacement.
Compare vids on Youtube of iPhone LCD screen replacement vs digitalizer replacement.