@scorpy77 Hi there, sorry to hear that you're leaving giffgaff for a better deal. I understand. Unfortunately, nothing is transferable between giffgaff accounts, therefore what you request is not possible. That's right, unused credit is not refundable.
If you are not going to use your credit and rather than letting it go to waste you could donate it to charity? Various charities accept donations by text. For example, giffgaff have a just giving page in support of Great Ormond Street Hospital here for you, via this link.
All the best with your new mobile network.