The Carphone Warehouse promotion is currently only available in store. However, Carphone Warehouse's sister company, mobiles.co.uk, is offering the same promotion online for Vodafone, T-Mobile and Orange PAYG.
It's likely that the promotion is aimed at helping shift a few more Lumia 710 units before the Nokia Lumia 610 becomes widely available.
The Lumia 610 is, subjectively, the more attractive device and has the "newer device factor" advantage, but it also has the lower Windows Phone specification (800Mhz processors with 256MB RAM) and is expected to have a launch price of around £169.99 PAYG. The Lumia 610 price will likely fall below that of the Lumia 710 in time, but right now the Lumia 710 is cheaper on the UK high street.
This makes the Lumia 710 (1.4GHz processor with 512MB RAM) rather good value for money. It's the ideal device for someone looking to get started with Windows Phone, but unwilling to stretch to a Lumia 800, Lumia 900 or HTC TITAN.
Here's our video review of the Nokia Lumia 710:
Thanks to misterc for the tip.