How about making buttons / links to request a GiffGaff Sim card and say view information about GiffGaff that are referral links.
For example I could Tweet "Check out GiffGaff's prices" and a link (i.e. giffgaff.com?ref=adamboutcher) and that goes to the info and I get say 10points
Where as I could also put "request a GiffGaff Sim" and a link (i.e. giffgaff.com/sim?ref=adambouctcher) and that goes to the sim request page which could give me 50 points, also this could mean when the sim card is activated I would get the 500 points from that too.
This would allow for more Internet links on blogs, tweets, websites etc.