Many users will not have taken the time to read Giffgaff's terms and Conditions. Being someone with no life I have plenty of time to do such a thing. One thing may Giffgaffers will not know is that they have signed up to telemarketing and such things. This is mentioned here:
My idea is provide a large Opt Out Button on the member's Home Page which will permit users to opt out of this "Marketing". It could collate and forward all the information required by Giffgaff's Data Controller. Giffgaff should be more open about this sort of thing and one way of doing this is to add that button.