I use GiffGaff myself, well pleased with what I get with it tbh.
I don`t have the "Goodybags" as i`m not a big talker/texter but if an event has happened where 1 month i`ve needed to use my mobile alot i`ve taken the £5 goodybag.
Check em out
www.giffgaff.com (non affiliate link)
Forgot to add all the kids are on giffgaff so free to talka nd text each other and we`ve found it`s free to ring/text o2 aswell (must be the same network)