Let's face it, the trade in are a bit of a rip off, so having said that, and if you want to get your hands on the standard 20, (this will work with other models including the A71 (£341))
You can get £100 trade in value for an iPhone 4 8GB, which you can buy at cex for £22 - £25 depending on condition.
This means you can get the standard s20 (dependant on the condition of your 4 you recieve, for £721 + £22 from topcashback, making the deal £699 all in, and if you don't have a very 20% off code
Links below to iPhone 4's from cex
Hope this helps.
Then you can sell your current phone to recoup some money back