The idea is this: Allow users who have their own web presence (blogs/websites/forums/etc.) to promote giffgaff using an affiliate link. Users who register via this will get users a small number of points (say, 50?) and users who actually activate their sims via this method get something similar to the normal activation prize (say 300 points?)
Should be very simple to create this functionality. (Stick an extra hidden variable in the registration form if the affiliate ID is present in the request URI).