How much would I have to pay for an iPhone 5 if I exchanged my 4 in?

Are there any reputable phone shops which will allow this? My phone was bought for a Christmas present (presumably from the apple store) and is in mint condition with boxing and earphones etc.I'm wanting to upgrade to a 5 because unfortunately the 8gb on my phone is just not enough, and the camera on the 5 is much better. I would be prepared to spend about £170-ish to upgrade as well exchange my 4 in. If anyone knows how much this would actually cost and if there are any places which do this your help would be greatly appreciated.

To buy iPhone 5 16GB on eBay
used - about £345
new - about £395

To sell iPhone 4 8GB on eBay
used - about £150