If you have the cash buying sim free and unlocked and then adding a sim only contract will be cheaper, if you haven't then GiffGaff are small up front payment with a minimal interest payment and give you the flexibility to do sim only or use them, good luck getting stock of anything worth having though. If you just want a handset with nothing to pay up front and want 4G (GiffGaff hasn't launched 4G despite the trial and is network throttling atm) then Tesco is reasonable relative tot he other options.