It's a scam
Please do not click on the link.
If you've already clicked on the link and completed any information, you should immediately:
Change your giffgaff password
Check your account details (name, address, email) have not been changed
Change your email password
Change your password on any other site that uses the same email/password combination (note: never use the same password more than once)
If your bank card is stored on your account, contact your bank to alert them to potential fraudulent activity