Right now you can grab the EE Buzzard from CPW half price for £19.99 delivered which gets you the device and also 2GB of 4G data.
Some important specs:
Supports up to 10 WiFi enabled devices at once
Dimensions: Size: 98x32x14.2mm
Plugs into your 12 volt power socket
(Thanks to TechNation @ HUKD for the tip)