Carphone Warehouse subsidiary E2Save is selling the LG Optimus 2X for £129.90 on Vodafone, excluding a compulsory £10 top-up. The handset is refurbished and is likely to be unlocked (as are almost all PAYG phones coming from CPW).
The Optimus 2X was launched back in January 2011 at CES in Las Vegas and was the first dual-core handset to hit the market. It ran on an Nvidia Tegra 2 system-on-chip clocked at 1GHz, with 512MB of RAM, 8GB onboard storage, a microSD card slot, a 4in 800 x 480 pixel display, an 8-megapixel camera, a VGA front-facing one, full HD video recording and Android 2.2 Froyo.
You should be able to upgrade the phone at least to Gingerbread and there are unofficial firmware upgrades that allow you to push the phone to Android 4.0 ICS. Check out our review of the LG Optimus 2X here.