I think I might have a solution.
Do you have any punctuation characters in either your name or address, such as a hash, full stop etc. The giffgaff validation stupidly rejects such chars without even giving an error message. I have posted an idea here about it and hoping that giffgaff sort it out soon. http://community.giffgaff.com/t5/Submit-Great-giffgaff-Ideas/Correct-the-Name-and-Address-validation...
The cure is simply to remove any of those types of characters and then it should accept your personal details as valid. Sorry I didn't notice this earlier.
If this is the problem, I'd be grateful if you update this thread so I can add it to my list of what trouble this is causing and get giffgaff to change it. Thanks