I had thought about leaving o2 Pay & Go because it seemed to be no longer very competitive compared to other networks in terms of the £10 bundles you can get out there. Called for PAC code and was surprised to be offered a retention deal. It's called the "Loyalty Big Bundle" and at £10 offers 2GB of data, 250 mins and 1000 texts. I know others offer unlimited texts for £10 but I think I'll struggle to get anywhere near 1000, so this was enough to convince me to stay. I mainly wanted a few more minutes because I wanted to stop paying extra for an inclusive call package on the landline, but I didn't want to pay £5 a month more with o2 (otherwise the savings from cancelling the landline package start to cancel themselves out)
I've been with o2 since 2003, and have previously had big bundles since summer 2016, not sure what the qualification is for other customers, but perhaps useful to know if you are thinking of jumping.