This data SIM from Three gives you 12GB over 12 months. It’s a full size SIM, but you can cut it down to micro-SIM size to use in an iPad if ye fancy.
The 3GB/3-month equivalent (£7.40 from Amazon) works out a little cheaper per GB, though with the 12GB version you don’t have to worry about expiry dates or peaky usage.
There's no need to activate or register the SIM. Just stick it in a device and the 3/12-month period starts automatically.
(Thanks to Petetobin on HUKD)