16GB PAYG 32GB PAYG SIM Price
O2 N/A £549.99 Free
Three £399.99 N/A Free
Google Play £299.99 £339.99 Free
Price Difference £100 £210 N/A

Buy the device from Google's Play Store (Carphone Warehouse is also offering the 16GB model for £295) and order a free pay-as-you-go SIM to go with it -- or you can, of course, use your own.

[Thanks, MartinS27!]