I went to The Carphone Warehouse a few weeks ago to upgrade from my S2 and saw this offer:
1. Buy the Sony Xperia Z2 on the deal you want, delivered to you or in store.
2. Trade in your old smartphone.
3. Once they receive your old smartphone, you'll get an extra £100 on top of the normal value of your old smartphone.
It's a great phone and the deal was pretty awesome.
I got £120 off my old phone (it was a bit scratched) which was a little more than paying for the new Sony Xperia.
Besides, if you like MJ, the new record comes with the phone and it's pretty decent.
Give it a go.