I always use Tesco vouchers for as much of the fare as I can. Even though I nearly always shop in Tescos and use their credit card all the time. I don't build up enough to pay for it all, and as you say I still have to pay for the dog on tunnel or ferry anyway.
I have given the tunnel a go, as I said above I tried it this year even though it cost more, and all the above comments are based on the stressful experience I had. I did not like it at all. Maybe it's like childbirth and eventually I will forget the nasty bits and only remember the good things about it ........... err, I stayed with the dog, (but he doesn't seem to mind being left in the van on the ferry anyway) but I can't think of any other good points. There are 3 years between my kids, so maybe I'll try again in 3 years.