Just bagged a couple of 3 Network 3 month sims off Amazon for £6.78. Look on the right and select the seller with the list price of £6.78. Can't be bad, works out at £2.26 per month.
I use mine in a MiFi dongle and never had any problems. I know some people have issues with 3 but as this is just a network connection on PAYG there's no contract issues to worry about.