Some users, particularly temporary visitors in the country like exchange students, order a giffgaff SIM, then have to throw it away after a few weeks or months of use only (see this thread for example).
Same with users disappointed in giffgaff, who switch to a different provider quickly.
Now I don't know how long SIM cards are designed to be used for (five years?) but this sounds like waste.
Could we imagine a scheme where people can "give back" their SIM card to giffgaff when they don't need it anymore? So giffgaff can have them re-used and sent to new users, instead of creating new ones?