If you want to keep a future SIM alive do any of the below within a 6 month period.
Make a least one call, SMS or MMS to another number. Note that emergency calls (i.e. 999/112) and calls or texts to giffgaff services (i.e. 2020,43431,43430) do not apply)
Make at least one connection to the internet via giffgaff
Make a least one topup/purchase (credit or goodybag)