I’m hunting around for a SIM only/PAYG deal at the moment, and giffgaff definitely looks like the way to go. A tenner per month gets you 250 minutes, unlimited data and unlimited texts.
The allowance lasts for one month. You can change your “goodybag” (giffgaff’s pet name for “tariff”) each month if you so desire. All calls and texts to other giffgaff members are free.
If you fancy more talk time, the same deal with 400 minutes is £15PM; 600 minutes is £20PM; 1,500 minutes is £25PM.
giffgaff uses the O2 network.