Yes, you need to register for a Tesco Clubcard. We have a Tesco credit /clubcard so all our purchases attract points.
In 2014 we had extensive alterations done to the house and I bought all the materials on the card, that's why I had so many points and was able to do two free journeys using the Tunnel. Originally it was four times the face value of the vouchers but that dropped to three times a couple of years ago.
They also can be used with DFDS ferries but when I checked this morning, there was no appreciable difference in the fares and as the Tunnel is so quick and convenient I decided to stick with them