I don't bother buying data anymore as there is free WiFi in so many places nowadays,I'd use that data in my lunch hour.
I can up it at any time (no need to wait till current months tariff ends) for £12 I'd get 2gb 3g or 4g, the other reason I don't need to spend a lot is my kids are also on giffgaff so calls & text to them are free, also 0800 numbers are free.
one last thing about giffgaff, if your willing to spend a hour or two on there forum each week helping out newbies you get given points, and twice a year they convert those points into £££, last year I made £78 for not a lot of work.