T-Mobile £10 per month SIM. It comes with 100 minutes and 100 texts but can also be bundled with a free unlimited internet flexible booster all for £10 a month. Great for a fairly light user that primarily wants internet access.
What sets it apart a bit is that it's only on a 30 day contract, is posted out free and qualifies for £80 Quidco.
Most other equivalent network SIM deals are around the £20 mark.
If you are after a phone then Argos are currently selling the Nokia 5230 for £74.99 which is already a good price and is full touchscreen and 3G and also comes with built in Sat Nav. So offset with Quidco you could end up with the phone costing £34.99.