There is an SMS scam going around targeting Giffgaff customers, which sends an SMS reading:
The request to swap your number to your new giffgaff SIM is pending. If that wasn't you, log in immediately. [URL removed]
If you go to the site, it looks like a giffgaff site and asks you to log in. It actually accepts any username and password, and then presents you with a screen asking you to confirm your personal details, including your bank account number.
This is a scam and has nothing to do with giffgaff. You should ignore the text message.