This is not good PR for the mobile sales team. As others have stated, as you have been given a full refund, there is no obligation for giffgaff to sell you another phone at the price you had purchased for. It would have been nice of them to do it though.
It may be worth messaging the complaints dept to make them aware. Try to keep it professional and explain how disappointed you were and how you will need to take your business elsewhere. Who knows, you may get something from your complaint. Worth a try, but remember, be nice, be professional and point out the areas you are dissappointed with and give constructive criticism as to how your would have preferred your problem to have been dealt with by giffgaff.
Hope you get a positive result - you got nothing to lose really - here is the link https://giffgaff.com/support/complaints