Have you checked if your Tesco clubcard points can be used in exchange for this ferry crossing as you get 4 times the face value of your clubcard vouchers. I'm saving them like mad!
If you come by car than Norfolk Line may be cheaper. It takes longer for the crossing but we saved loads of money last year going to Crufts. Norfolk Line also accepts 20% off if member of IKC, maybe they do the same for UK kC?