As I have gone to great trouble to explain, you can opt out.
The Electoral Register is a public document; you can go into the library and ask to look at it. Always has been. Selling public data is easier for Councils and purchasers, such as political parties and credit check agencies. They're the big buyers - not Mickey Flood who's selling pizzas down the road.
Personally I don't have a Tesco Clubcard. Never have had. They know more about you than the government, yet millions have them and other, similar loyalty cards and if you don't tick that tiny little box they'll sell your data too. Maybe you have one too? So it's hardly outrageous, is it?