I've pasted below my answer to a similar question from another forum. You can convert your Tesco points to Airmiles.
Right, I've just phoned Airmiles and they were happy to explain how pricing works for Airmiles to Santander and for me to post this.
Estimate for Portsmouth to Santander, depart Mon 8 June 2010, Kia Sorento + Wyoming, 2 passengers, standard cabin and return on Monday 28 June.
Total cash price = £1,043. You can get this by going to the Brittany Ferries site.
Pay wholly with Airmiles = 17,000 Airmiles
To use part Airmiles and remainder in cash, as a rough guide, 500 Airmiles = around £30.
That's next year's holiday for us