Nanogaff is a community-powerds distribution system for GiffGaff Nano SIM cards.
This is the link where you can order your Nano SIM: http://nanosim.giffgaffapps.com
Here is also the link on how to swap you standard or micro SIM to the Nano SIM, via the SIM swap procedure:
http://community.giffgaff.com/t5/Learn-giffgaff-Top-Tips/What-is-SIM-Swap/ta-p/7320084