Cashback and/or rollover of unused allowances have been the subject of many ideas and rejected by giffgaff, here is an explanation from a previous idea:
Status: Not For Us
[11th Feb 2015]: This idea has been set to 'Not For Us'. Hi everyone,
We've been having in depth discussions about this for a while due to it's tremendous support. This has been a genuinely record breaking idea for the amount of Kudos it's received in such a short time. Ultimately we've had to hold up our hands and say, despite this being giffgaffy and something we would love to do for our members, this can't be done. After running the numbers, it's not financially viable and would actually add complexity to our offerings. Part of the attraction of goodybags is the freedom to use what you want, and we'd be concerned about adding pressure onto members to not use their phone, or pay for more than they need if they expect to get cashback.
Thanks for your support on this, please don't hestiate to post ideas like this in future as we really appreciate the thought, effort and desire to make giffgaff better.