HI there @gretelaane - a sim card can be posted outside of the UK yes. If you have lost your sim card you need to order a replacement using the link below........
SIM Swap guide.
giffgaff SIMs are now 'All in one SIMs' and the SIM card contains a Nano, Micro and a Standard [mini] SIM. Have a check of your SIM, you may not need to swap at all. Just pop out the section that you need.
An image of the new 3-1 SIMs