I'm not sure if this has been posted before but I've been using this for the past year and I'm sure someone will benefit from it.
Essentially I brought a Three PAYG SIM card from P4U instore for £10 and on it comes preloaded with £30 credit. I change the preloaded credit to 1Gb worth of data which cost £10, which I use on my own dongle.
From the preloaded £30 credit you can get 3 months of 1Gb data per month, which for me is more than enough as I use it for only e-mails.
I've only ever brought the Three PAYG SIM card from P4U instore, not sure if they're the same else where.