TPO who piggy back off of EE are doing until 31/12/14 unltd calls & texts and 3GB of data for 3.99 a month for the first 3 months, then reverts to 14.99 a month.
Plus 10% of your net spend goes to a charity of your choice at no extra cost. I ported into TPO a couple of months back with no issues, good 3g coverage here. Still no word on 4g yet from them and no tethering.
Dont forget TopCashBack as well (ten pounds I think)